Report on the Radiolaria/Plectoidea

by Ernst Haeckel

Suborder II. PLECTOIDEA, Haeckel.

Plectoidea vel Plectida, Haeckel, 1881, Prodromus, p. 423.
Plagiacanthida (sensu ampliori), R. Hertwig, 1879, Organismus d. Radiol., p. 72.

Definition.—Nassellaria with a rudimentary, originally tripodal skeleton, composed of radial spines, arising from one common central point or central rod; the spines are simple or branched, and the branches may form by concrescence of their meeting ends a loose wickerwork, but never a complete lattice-shell. Never a ring in the skeleton.

The suborder Plectoidea, hitherto known by few species only of "Plagiacanthida," comprises a large number of interesting Nasselaria, which belong partly to the simplest and most primitive forms of this legion. It may be divided into two different families, Plagonida and Plectanida. In the first family, Plagonida, the monopylean central capsule is supported by a simple or rudimentary skeleton, composed only of a variable number of radial spines united in a common centre. In the second family, Plectanida, the branches of these radial spines become united and form a loose irregular framework with wide meshes, partly enclosing the central capsule, but never a perfect lattice-shell.

The Plectoidea differ from the following suborder, the Stephoidea, in the absence of the ring, characteristic of the latter. Some slight traces, however, indicate a near affinity between the ringless Plectoidea and the ring-bearing Stephoidea. Both these suborders of Plectellaria differ from the closely allied Cyrtellaria (Spyroidea, Botryodea, and Cyrtoidea) in the absence of a complete lattice-shell. The morphological relation and phylogenetic affinity between the former and the latter have already been discussed in the preceding description of the legion Nassellaria (compare pp. 891-894).

The first known species of Plectoidea was observed in the North Atlantic (on the Norwegian shore) in 1855 by my late friend Edouard Claparède, and described and figured in his Études, &c. (1858), under the name Plagiacantha arachnoides. He considered it as a new genus of Acanthometrina. Another species, from the Mediterranean, was described in the same year by Johannes Müller as Acanthodesmia dumetum (1858, loc. cit., Taf. i. fig. 3). A third species, also Mediterranean, was figured by me in 1865 under the name Acanthodesmia polybrocha. Finally, Richard Hertwig, 1879, in his Organismus der Radiolarien, gave a very accurate description of another Mediterranean form, Plagiacantha abietina (loc. cit., Taf. vii. fig. 6). He first recognised the true character of Monopylea in their monaxonian central capsule, and observed at the same time the first Nassellarium without skeleton, called by him Cyrtidium inerme (loc. cit., Taf. vii. fig. 1). To these four known species, representing three different genera, the rich collection of the Challenger has added so many new forms that we may distinguish here not less than seventeen genera and sixty-one species. In my Prodromus (1881, p. 423) I arranged these in two subfamilies, the Plagonida and Plectanida, constituting together the family Plectida (identical with the "Plagiacanthida" of Hertwig and Bütschli). But at present, regarding the important relations of these Plectida to the other Nassellaria, it seems more convenient to give to them the rank of an independent suborder of Radiolaria, under the name Plectoidea.

The peculiar structure of the central capsule of the Plectoidea, first recognised by Richard Hertwig, allows no doubt of their being true Monopylea or Nassellaria; and also their siliceous, originally triradiate skeleton indicates the nearest affinity to the other families of this legion. But a very difficult and as yet unsolved problem is the important question, in what manner these different groups of Nasselaria are phylogenetically connected. Either the Plectoidea—as the simplest of all—are the original common ancestral group of this whole legion (as I assumed in my Prodromus, 1881), or they are derived from the Stephoidea (by reduction of the sagittal ring), or they have originated independently from them (if we suppose a polyphyletic origin of the Monopylea. Compare above, p. 893, &c.). In any case the typical "triradial structure" of the Plectoidea, prevalent also in the other groups of this legion, is a very important and interesting fact.

The triradial skeleton of the Plectoidea exhibits in the two families of Plagonida and Plectanida a complete homology of development, so that each genus of the latter may be derived from a corresponding genus of the former, simply arisen by concrescence or union of the branches of the radial spines. Therefore the only difference between the two closely allied families is, that the branches of the radial spines in the Plagonida remain free, whilst in the Plectanida they produce a loose framework or wickerwork by union of their meeting ends. We express this complete homology in the nomenclature of the Plectoidea, in each genus of Plagonida retaining the syllable "Plag-"; in each genus of Plectanida, correspondingly, the syllable "Plect-."

The number of radial spines composing the skeleton is originally three, and in all not triradial genera is probably derived from three. For better survey we may divide each family, according to the different number of rays, into four different subfamilies: A, with three radial spines (Triplagida and Triplectida); B, with four radial spines (Tetraplagida and Tetraplectida); C, with six radial spines (Hexaplagida and Hexaplectida); and D, with numerous (seven to nine or more) radial spines (Polyplagida and Polyplectida). The last three subfamilies have arisen probably from the first triradial subfamily, by a secondary increase in the number of rays.

The important signification of the triradial structure, recurring in the most different groups of Nassellaria, has been already pointed out sufficiently by myself and by R. Hertwig. But the triradial Plectoidea offer also another interesting relation of this characteristic structure, some simple forms of this order appearing nearly identical with the isolated triradial spicula of certain Beloidea (Thalassosphærida and Sphærozoida). Even some more complex quadriradial and sexradial forms of the latter reappear in exactly the same shape also in the former. This identity may be perhaps an important indication of true affinity (compare below).

The simplest and probably the most original kind of triradial structure is exhibited by the genera Triplagia and Triplecta (Pl. 91, figs. 2, 7). Here three equal radial spines lie in one horizontal plane and are united in a common central point at equal angles, so that three lines connecting their distal ends form a regular equilateral triangle. Simple triradial spicula of the same regular form are also found in many Beloidea (Lampoxanthium, Sphærozoum, &c., Pls. 2 and 4). The central capsule of these simplest Plectoidea (with vertical main axis) rests perpendicularly on the horizontal triangle, formed by the triradial skeleton; the porochora of the former (or the "area porosa") rests upon the central point of the latter.

Another kind of triradial structure characterises the genera Plagiacantha and Plectophora. The three radial spines united in the central point lie here not in one plane, but diverge in different planes, so that they correspond to the three lateral edges of a three-sided pyramid. Commonly the three spines are of equal size, and also the angles between them equal, so that the pyramid is regular, sometimes very flat, at other times more elevated. Spicula of exactly the same form are also found in some Beloidea. Probably the three divergent spines are homologous to the three basal feet of numerous Spyroidea and Cyrtoidea. The central capsule, according to Hertwig, is placed in the apical part of the pyramid, the axes of both being identical, and the porochora resting in the apex itself. This fact seems to contradict the above-mentioned affinity; but since in Triplecta and Triplagia the three spines lie horizontally, they may have changed this original position in different direction, in Plagiacantha and Plectophora becoming divergent upwards, whereas in Plagoniscus and Plectaniscus (as in the Spyroidea and Cyrtoidea) directed downwards.

The triradial structure, common to the Triplagida and Triplectida, is replaced by the quadriradial structure in the Tetraplagida and Tetraplectida. Probably the latter have been derived from the former by development of a fourth spine, and then this latter would correspond to the "apical horn" of the other Nassellaria. But possibly also both structures have originated independently from one another. We may distinguish not less than four different kinds of the quadriradial structure. In the first case all four spines are equal, and diverge from a common central point at equal angles in different directions, corresponding to the four axes of a regular tetrahedron (Tetraplagia and Tetraplecta, Pl. 91, figs. 3, 8).

In the second case all four spines are also equal, but they are not united in a common central point, but opposite in pairs on the two poles of a common central rod (Plagonidium). Therefore the skeleton possesses here the same form as in the "geminate-biradiate" spicula of many Beloidea (e.g., Thalassoxanthium bifurcum and Sphærozoum furcatum). The development of the short horizontal middle rod, connecting the two divergent pairs of spines, is here probably effected by the porochora of the central capsule resting upon it.

Whilst in these two cases of quadriradial structure all four spines are equal, in two other cases they become differentiated in a very remarkable manner. One spine is vertically directed upwards, in shape and size different from the three others, which are directed downwards; the former corresponding probably to the "apical horn," the latter to the three "basal feet," which are found in the great majority of the Spyroidea and Cyrtoidea. Therefore we encounter here for the first time that characteristic "cortinar structure" which is complete in Cortina and Cortiniscus (Pl. 92, figs. 11-13, 21), and which may be regarded as the strongest argument for a close relationship, or even for a common monophyletic origin of all Nassellaria.

The four spines, which we regard therefore as "cortinar spines," exhibit a twofold kind of central junction. In the simpler case they are united in a common central point, on which rests the porochora of the central capsule (Plagoniscus and Plectaniscus, Pl. 91, figs. 4, 9). These forms are nearer to Cortina, and may be derived immediately from Tetraplagia and Tetraplecta by differentiation of the four equal spines. In the other case the four cortinar spines are separated in pairs, diverging from the two poles of a short horizontal common "central rod" (Plagiocarpa and Periplecta, Pl. 91, figs. 5, 10). These forms may be compared with the spicula of some Beloidea and derived from Plagonidium; but their basal central rod may be compared again with the basal part of the sagittal ring of Cortina, and this comparison becomes very important in those forms like Plagiocarpa procortina (Pl. 91, fig. 5). Here the four spines approach very nearly to those of Cortina; the two ventral spines (or pectoral feet) on the anterior pole of of the middle rod are equal, but very different from the two dorsal spines, arising from the posterior pole; the lower odd spine of the latter corresponds to the "caudal foot," the upper spine to the "apical horn" of Cortina and of the Cyrtellaria. The vertical plane, determined by these two dorsal spines, is the sagittal plane, and two opposite curved branches which lie in this plane (an upper arising from the basal part of the apical spine and a lower arising from the anterior pole of the middle rod) may be regarded as ventral parts of an incomplete sagittal ring. This interesting form and some other similar Tetraplagida may be regarded either as beginning Stephoidea (Cortina, with incomplete sagittal ring) or as retrograde Stephoidea (Cortina, with partly reduced sagittal ring). In every case they seem to indicate the near relationship between the Stephoidea and Plectoidea.

Another argument for this close relationship may be found in the position of the central capsule in the interesting genus Plagiocarpa (Pl. 91, fig. 5). Its basal part (with the porochora) rests upon the common central rod, its ventral face upon the ventral prolongation of the latter, its dorsal face upon the apical horn; its axis lies in the sagittal plane. The three basal spines (the odd caudal and the paired pectoral feet) diverge from its basal pole downwards in the same manner as in the Cortina, the Zygospyrida and the Monocyrtida.

Less important than those quadriradial Tetraplagida and Tetraplectida, are the sexradial Plectoidea, the Hexaplagida and Hexaplectida. These may be derived immediately from the triradial Plectoidea by prolongation of the three primary original spines (of Plagiacantha) over the common central point. Here also two different kinds of central junction are found. In the simpler case all six radial spines arise from a common central point (Hexaplagia and Hexaplecta). In the other case the six radial spines arise from the two poles of a short horizontal common central rod, opposed in two groups, each of three spines (Plagonium and Plectanium, Pl. 91, figs. 6, 11). In this latter case the single corresponding spines of the two opposite groups are usually parallel, and exhibit therefore exactly the same characteristic "germinate-triradiate" form which is found in many Beloidea (e.g., in the common Sphærozoum punctatum and the similar Lampoxanthium punctatum).

The fourth and last group of this suborder contains the multiradiate Plectoidea, the Polyplagida and Polyplectida. Here the number of radial spines, diverging from the common centre, exceeds six, and is commonly seven to nine, at other times ten to twelve or more (Polyplagia and Polyplecta, Pl. 91, fig. 12). When these two genera are better known from further accurate observations, they may probably be divided into several different genera (as already proposed in my Prodromus, 1881), since not only the number, but also the central junction and the arrangement of the numerous radial spines in the few observed species seems to be very different. In some seven-radiate species (e.g., Polyplecta heptacantha) four larger spines seem to be true "cortinar spines," the three smaller secondary productions of the former. In the nine-radiate species the nine spines seem to be sometimes basal branches of three primary spines, at other times six secondary intercalated between the three primary (like Enneaphormis, Pl. 57, fig. 9). In those multiradial Plectoidea, in which the number of spines amounts to ten or twelve or more, the laws of disposition are not yet recognised.

Comparing these different productions of the skeleton in the numerous Plectoidea, we find expressed two remarkable and very different affinities. On the one hand many Plectoidea exhibit exactly the same peculiar forms, which are only found besides in the Beloidea (as many species of Triplagia, Plagiacantha, Tetraplagia, Plagonidium, Hexaplagia, and Plagonium). On the other hand many Plectoidea bear the same characteristic composition of the skeleton (or the "cortinar structure") which is found in the Cortinida among the Stephoidea, and in numerous Spyroidea and Cyrtoidea, which all agree in the possession of three divergent basal feet and a vertical apical horn. A most important argument for the close affinity of all these "cortinar Nassellaria" seems to be given by the fact that the sagittal ring, which in Cortina is combined with the quadriradial structure, exhibits in the Cyrtellaria the most different stages of development; in one group it is complete, in the second incomplete, and in the third it has completely disappeared.

The form of the radial spines composing the skeleton is usually three-sided prismatic, gradually tapering from the thicker central base towards the distal apex; sometimes they are slender pyramidal. More rarely the spines are cylindrical or slender conical. In the majority of species the spines are straight, in the minority more or less curved. In very few species only are they quite simple, without branches. They are nearly always more or less branched, in many larger species very richly ramified. The modes of ramification are rather variable. In the majority of Plectoidea the spines are rather regularly verticillate, bearing an increasing number of verticils, each of which is composed of three divergent branches. These arise from the three edges of the spine, and all the branches of one edge are usually parallel, either perpendicular to the spine, or directed at an acute angle towards its apex. When the verticils are numerous (five to ten or more), their size commonly tapers gradually towards the apex. Pinnate spines occur more rarely than verticillate ones; in this case the two paired lateral edges only of the prismatic spine bear opposite or alternate branches, whilst the odd middle edge bears no ramules. In some species the spines are singly or doubly forked. In many species (mainly those with cylindrical spines) the ramification of the spines is more or less irregular.

Whilst in all Plagonida the branches of the spine remain perfectly free, in all Plectanida, again, the meeting ends of the branches become united and grow together, and by this concrescence a loose network arises, like wickerwork, which partly encloses the central capsule and the central parts of the spines, on which it rests. The meshes of this loose wickerwork are large, either quite irregular, of very different size and form, or more or less regular, with a certain form and arrangement of the meshes, effected by the peculiar kind of ramification. Commonly the siliceous threads of the arachnoidal wickerwork are very thin, often extremely delicate, representing "pseudopodia metamorphosed into silex." Sometimes the wickerwork is spongy. Its surface is constantly rough and bristly, with free ends of the spine-branches, never covered with a regular lattice-plate, as in the Cyrtellaria (Spyroidea, Botryodea, and Cyrtoidea).

The entire form of the central wickerwork is in the minority of Plectanida quite irregular and indefinite; in the majority, however, a certain more or less regular entire form is recognisable, effected by a certain, more or less regular origin and mode of the connection of the meeting branches. So in some species of Triplecta (Pl. 91, fig. 7) the network represents a triangular plate, of Plectophora and Plectaniscus a three-sided pyramid, of Tetraplecta (Pl. 91, fig. 3) a tetrahedron, and in many other species a polyhedron of more or less regular form. Some species of Plectanida become very similar to certain species of Stephoidea, Spyroidea, and Cyrtoidea; so Plectaniscus and Periplecta approach to Cortina and Cortiniscus, Pteroscenium and Clathrocorys, &c. (compare Pls. 92, 93, 53, 64, &c.). They may represent a true phylogenetic connection between both groups. But in these cases also the distinction is determined by the fact that the true Plectoidea never possess a complete sagittal ring (like the Stephoidea) nor a regular lattice-shell (like the Spyroidea, Botryodea, and Cyrtoidea).

The Central Capsule of the Plectoidea constantly exhibits the peculiar characters of the Monopylea or Nassellaria. It is commonly ovate, more rarely ellipsoidal or even spherical, sometimes conical or lentelliptical. The lower or basal pole of its vertical main axis constantly exhibits the characteristic "porochora" (or the area porosa) of the Monopylea, and upon this rests the peculiar "podoconus" (or the pseudopodial-cone) of this legion. On this porochora the central capsule is in immediate connection with the central point of the skeleton, or the horizontal common central rod, from which the radial spines arise. The endoplasm, or the protoplasm of the central capsule (besides the podoconus), contains commonly one large alveole or several small vacuoles, and often pigment-granules. The nucleus is large, spherical or ovate, and exhibits the same character as in all the other Monopylea; it encloses usually a single nucleolus.

The position of the central capsule and its topographical relation to the skeleton offers in the different Plectoidea some important and as yet unsolved problems, which can be answered only by fresh and accurate observations on living specimens. In Triplagia and Triplecta, where the triangular skeleton lies in a horizontal plane, the vertical main axis of the central capsule is perpendicular to the central point of that supporting triangle. In Plagiacantha and Plectophora, where the three radial spines correspond to the edges of a flat pyramid, the capsule is enclosed in the pyramidal space of the latter, its basal pole touching the apex; therefore in the normal position of the body the three divergent rays are directed upwards. In Tetraplagia and Tetraplecta probably the same position is retained, and therefore the fourth free spine, here developed, is probably directed vertically downwards. In Plagoniscus and Plectaniscus, however, and moreover, in the closely allied Plagiocarpa and the corresponding Periplecta (Pl. 91, figs. 5, 10) the position of the central capsule, relative to the skeleton, seems to be inverse, and to agree with that of the Stephoidea (Cortina, Cortiniscus, &c.) and the Cyrtoidea (Pteroscenium, Clathrocorys, &c.); the three divergent spines are here directed downwards (as basal feet), whilst the opposite fourth spine is vertically directed upwards (as an apical horn); the capsule rests here upon the tripod, which lies below it, and is inclined with its dorsal face to the apical spine. In the majority of the other Plectoidea the position of the central capsule and its relation to the skeleton are not yet sufficiently observed, and require further accurate researches. Its position seems to be very different in the several genera. The capsule is never perforated by parts of the skeleton; this latter is constantly extra-capsular.

The physiological value of the skeleton, with regard to the central capsule, is different in the Plagonida and Plectanida; in the former it supports, in the latter it encloses the capsule like a shell. In the Nassellida, where no skeleton is developed, the central capsule is quite free and naked, enveloped only by the calymma.

The calymma or the extracapsular jelly-veil in all Plectoidea is voluminous, and encloses not only the central capsule completely, but also the skeleton wholly or partially. Its form is of the greatest value for the development and configuration of the skeleton. Sometimes the calymma is alveolate and foamy, as in Nassella and the common Thalassicolla. In several other Plectoidea the calymma seems to include numerous small vacuoles, sometimes also pigment-granules. Xanthellæ are commonly scattered in it in great numbers. The pseudopodia, arising in a large bunch from the porochora of the capsule, and running along the branches of the radial spines, seem to be always numerous, richly branched, and with a strong tendency to form anastomoses. The peculiar form of their network is often exactly preserved in the conformation of the skeleton, produced by them. The peculiarities of this network require further accurate observations, as does the whole organisation of the Plectoidea.

Synopsis of the Families of Plectoidea.

I. Skeleton (originally tripodal) composed of radial spines united in a common centre and supporting the central capsule, without wicker-work,	1. Plagonida.
II. Skeleton (originally tripodal) wattled, with irregular wicker-work, composed of the united branches of radial spines and enveloping the central capsule,	2. Plectanida.

Family XLVI. Plagonida, Haeckel.

Plagonida, Haeckel, 1881, Prodromus, p. 423.
Plagiacanthida (sensu strictiori), Richard Hertwig, 1879, Organismus d. Radiol., p. 72.

Definition.—Plectoidea with a spiny skeleton, composed of radial spines, which arise from a common central point or central rod, and support the free central capsule.

The family Plagonida comprises those Nassellaria in which the skeleton is only composed of united radial spines, arising from a common centre, without any connection of meeting branches of the spines; the rudimentary skeleton exhibits therefore neither a loose wickerwork (as in the closely allied Plectanida), nor a ring (as in the Stephoidea), nor a complete lattice-shell (as in the Cyrtellaria, the Spyroidea, Botryodea, and Cyrtoidea). The central capsule, which possesses all the characters of the Monopylea, is therefore free, not enclosed, and only on one side supported or partly protected by the radial spines or their branches.

Two species only of Plagonida have been hitherto known. The first form described is the Plagiacantha arachnoides, discovered thirty years ago (1855) by Claparède on the western shore of Norway. Another species of the same genus, from the Mediterranean, was very accurately described by Richard Hertwig in 1879 in his Organismus der Radiolarien (Plagiacantha abietina). Upon these two species the latter founded his family Plagiacanthida, a term which was afterwards employed by Bütschli and others, for the whole group of Plectoidea. Many new forms are contained in the collection of the Challenger, so that we may describe here nine genera and thirty-four species.

The family Plagonida may be divided into four different subfamilies, according to the numbers of the radial spines which compose the skeleton: Triplagida with three, Tetraplagida with four, Hexaplagida with six, and Polyplagida with numerous (seven to nine or more) radial spines. These are united commonly in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera Plagonidium, Plagiocarpa, and Plagonium) the spines arise in two opposite groups (each with two or three spines) from the two poles of a common central rod; in this case the basal pole of the central capsule with the porochora rests upon the horizontal common rod, which corresponds probably to the basal part of the sagittal ring of the Stephoidea and Cyrtellaria.

The different forms which the skeleton of the Plagonida assumes in the different genera of this family, and the important relations which these exhibit on the one hand to the spicula of the Beloidea, and on the other hand to the shell of some Stephoidea (Cortina, Cortiniscus, &c.) and Cyrtoidea (Pteroscenium, Clathrocorys, &c.), have been already pointed out in the preceding description of the suborder Plectoidea. There it is also demonstrated, that all these different forms may be derived from the simplest triradial forms, Triplagia and Plagiacantha (compare above, pp. 900-904).

Whilst the genera of the Plagonida are characterised by the number of the radial spines and the peculiar mode of junction in a common central point or at the two poles of a common central rod, the different species of this family may be defined by the peculiar form of the spines and their branches. These morphological characters have also been already described above. We repeat here only that the radial spines in the majority of species are three-sided prismatic and verticillate, each verticil commonly with three branches. The distal ends of these branches remain constantly free, and are never united, as is always the case in the following family.

The Central Capsule of the Plagonida exhibits the general characters of all Monopylea (compare above, p. 890). It is commonly ovate or ellipsoidal, with vertical main axis; on the lower pole of the latter is the porochora (or the "area porosa," from which all pseudopodia radiate). This is in immediate connection with the central point or central rod of the skeleton, in which its radial spines are united. The topographical relation of the supporting skeleton to the central capsule seems to exhibit in the different genera of the Plagonida remarkable differences, as already demonstrated above (p. 905).

Synopsis of the Genera of Plagonida.

I. Subfamily Triplagida. Three radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Three spines lying in one horizontal plane,			384. Triplagia.
		Three spines corresponding to the edges of a flat pyramid,			385. Plagiacantha.
II. Subfamily Tetraplagida. Four radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Four spines arising from one common central point.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	386. Tetraplagia.
				One apical spine opposed to three basal spines,	387. Plagoniscus.
		Four spines arising in two pairs from the poles of a common central rod.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	388. Plagonidium.
				One apical spine opposed to three basal spines,	389. Plagiocarpa.
III. Subfamily Hexaplagida. Six radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Six spines arising from one common central point,			390. Hexaplagia.
		Six spines arising in two opposite groups from the poles of a common central rod,			391. Plagonium.
IV. Subfamily Polyplagida. Numerous radial spines.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod),			392. Polyplagia.

Subfamily 1. Triplagida, Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with three radial spines.

Genus 384. Triplagia,^[1] Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus Triplagia and the following closely allied Plagiacantha may be regarded as the simplest and most primitive forms of the Plectoidea, perhaps as the common ancestral stock of this suborder. The skeleton is composed of three simple or branched radial spines, arising from one common central point. These three spines in Triplagia lie in one and the same plane, whilst in Plagiacantha they lie in different planes. Therefore the former exhibits the simplest type of the triradial structure, common to the majority of Nassellaria.

1. Triplagia primordialis, n. sp. (Pl. 91, fig. 2).

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, each with two pairs of opposite lateral branches, which are correspondingly parallel to the two other spines; the proximal branches twice as long as the distal branches.

Dimensions.—Length of each spine 0.2, of the basal branches 0.07.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

2. Triplagia triradialis, n. sp.

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, regularly pinnate, with six pairs of opposite pinnulæ, tapering gradually towards the distal ends; the basal pinnulæ are again branched, with straight ramules.

Dimensions.—Length of each spine 0.27, of the basal branches 0.08.

Habitat.—North Pacific, Station 256, surface.

3. Triplagia horizontalis, n. sp.

Spines curved, cylindrical, irregularly branched, with three primary and nine to twelve secondary branches, which are also slightly curved. The angles between the bases of the three spines are equal, but the size and form of the branches different.

Dimensions.—Length of the three spines 0.15 to 0.25, of the basal branches 0.05 to 0.08.

Habitat.—West Tropical Pacific, Station 226, depth 4475 fathoms.

Genus 385. Plagiacantha,^[2] Claparède, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, November 13.

Definition.—Plagonida with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus Plagiacantha agrees with the preceding Triplagia in the simple structure of the triradial skeleton, composed of three diverging radial spines, which are united in a common centre at the oral pole of the central capsule. But whilst the three radial rods of Triplagia lie in one horizontal plane, here they lie in different planes and correspond to the three edges of a flat pyramid. Plagiacantha arachnoides, described in 1856 by Claparède, was the earliest known form of all Plectoidea.

1. Plagiacantha arachnoides, Claparède.

Plagiacantha arachnoides, Claparède, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, November 13.

Plagiacantha arachnoides, Claparède, 1858, Études sur les Infusoires et les Rhizopodes, p. 462 (pl. xxii. fig. 8).

Acanthometra arachnoides, Claparède, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 675.

Spines straight, cylindrical, divided into three divergent straight branches of equal size; each branch two to three times as long as the simple thicker basal part.

Dimensions.—Length of the spines 0.12 to 0.2 of the branches 0.08 to 0.14.

Habitat.—North Atlantic, coast of Norway, Claparède, surface.

2. Plagiacantha furcata, n. sp.

Spines straight, cylindrical, divided into two divergent straight branches of equal size, of about the same length as the simple basal part. The spines and their branches are smooth.

Dimensions.—Length of the spines 0.24, of their fork-branches 0.12.

Habitat.—North Pacific, Station 254, surface.

3. Plagiacantha dodecantha, n. sp.

Spines straight, three-sided prismatic, in the middle with one verticil composed of three divergent straight branches of equal size, of about the same length as the simple basal part. The edges of the spines and their branches are thorny.

Dimensions.—Length of the spines 0.24, of the branches 0.1.

Habitat.—Central Pacific, Station 274, surface.

4. Plagiacantha abietina, Richard Hertwig.

Plagiacantha abietina, Richard Hertwig, 1879, Organismus d. Radiol., p. 72, Taf. vii. figs. 6-6b.

Spines straight, three-sided prismatic, with three verticils of three divergent straight lateral branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the second, and four times as long as those of the third verticil. All nine branches of each spine simple, straight, three-sided pyramidal.

Dimensions.—Length of the spines 0.2 to 0.3, of the basal branches 0.06 to 0.08.

Habitat.—Mediterranean, Messina (R. Hertwig), surface.

5. Plagiacantha verticillata, n. sp.

Spines curved, three-sided prismatic, with six to nine verticils of three divergent branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the fourth verticil. All branches simple, slightly curved, bristle-shaped.

Dimensions.—Length of the spines 0.27, of the basal branches 0.12.

Habitat.—South Pacific, Station 296, surface.

6. Plagiacantha elatine, n. sp.

Spines straight, three-sided prismatic, with prominent wing-like edges and ten to twelve verticils of three divergent branches, decreasing in size towards the distal end; the branches of the three or four basal verticils are again ramified and much larger than the simple branches of the distal verticils.

Dimensions.—Length of the spines 0.36, of the basal branches 0.14.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

Subfamily 2. Tetraplagida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four radial spines.

Genus 386. Tetraplagia,^[3] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four equal radial spines, arising from one common central point, and corresponding to the four axes of a tetrahedron.

The genus Tetraplagia is one of the most important Plectoidea, and possibly the common ancestral form of many Nassellaria (compare above, p. 901). The skeleton is composed of four radial rods, diverging from one common centre in different directions, and corresponding to the four axes, which extend from the central point of a tetrahedron to the central points of its four faces. The whole skeleton of Tetraplagia exhibits therefore the same form, which is observed in the single quadriradiate spicula of some Beloidea (in some species of Lampoxanthium, Sphærozoum, &c.).

1. Tetraplagia geometrica, n. sp.

Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the four geometrical axes of a regular tetrahedron; in the middle arise from each spine two opposite equal lateral branches of half the length of the spine.

Dimensions.—Length of the spines 0.12, of the branches 0.06.

Habitat.—Central Pacific, Station 266, surface.

2. Tetraplagia phænaxonia, n. sp. (Pl. 91, fig. 3).

Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising from the three edges; the branches are thorny, tapering towards the apex.

Dimensions.—Length of the spines 0.15, of the basal branches 0.03.

Habitat.—Tropical Atlantic, Station 332, surface.

3. Tetraplagia abietina, Haeckel.

Plagiacantha abietina, var. quadrispina, Richard Hertwig, 1879, Organismus d. Radiol., p. 73.

Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple straight branches; the branches of each edge are parallel, tapering towards the distal end. R. Hertwig regards this species only as a four-spined variety of his three-spined Plagiacantha abietina; but a specimen, observed by me in Corfu, exhibited all the characters of Tetraplagia.

Dimensions.—Length of the spines 0.2, of the basal branches 0.07.

Habitat.—Mediterranean (Messina, Corfu), surface.

Genus 387. Plagoniscus,^[4] n. gen.

Definition.—Plagonida with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.

The genus Plagoniscus agrees with the preceding Tetraplagia (its probable ancestral form) in the possession of four radial spines, diverging from one common central point. But whilst in this latter all four spines are equal, corresponding exactly to the four axes of a tetrahedron, here in Plagiocarpa an important difference exists between one vertical or apical spine and three other divergent basal spines; these latter corresponding probably to the three "feet," the former to the single "apical horn" of the majority of Nassellaria. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. 902).

1. Plagoniscus tripodiscus, n. sp. (Pl. 91, fig. 4).

Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat shorter, curved, with three thorny edges.

Dimensions.—Length of the apical spine 0.2, of the basal spines 0.15.

Habitat.—Central Pacific, Station 263, surface.

2. Plagoniscus euscenium, n. sp.

Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved basal spines, each of which bears three verticils; the basal verticils larger and ramified. Resembles somewhat Euscenium eucolpium, Pl. 53, fig. 12, but has no latticed shell.

Dimensions.—Length of the apical spine 0.3, of the basal spines 0.16.

Habitat.—North Pacific, Station 247, surface,

3. Plagoniscus cortinaris, n. sp.

Spines three-sided prismatic, straight, verticillate. Apical spine with twelve verticils, one and a half times as long as the three basal spines, each of which bears eight verticils of three branches. The branches are also prismatic, straight, on each edge parallel, tapering towards the apex, in the three basal spines forked, in the apical spine more branched. Similar to the cortinar skeleton of Clathrocorys (Pl. 64, figs. 8-10).

Dimensions.—Length of the apical spine 0.24, of the basal spines 0.16.

Habitat.—South Atlantic, Station 338, surface.

4. Plagoniscus nassellaris, n. sp.

Spines cylindrical, curved, irregularly branched. Apical spine half as long as, and less branched than the three basal spines, which are sigmoidal, nearly horizontally expanded in the proximal half, descending in the distal half.

Dimensions.—Length of the apical spine 0.1, of the three basal spines 0.18.

Habitat.—West Tropical Pacific, Station 224, surface.

Genus 388. Plagonidium,^[5] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four equal radial spines, arising in pairs from the two poles of a common central rod.

The genus Plagonidium, and the following Plagiocarpa possess four radial spines, like the two preceding genera. But whilst the four rods in these latter arise from a common centre, here they arise in two pairs from the two poles of a common middle rod; they have therefore exactly the same form which we find in the single spicula of some Beloidea (e.g., Thalassoxanthium furcatum, Sphærozoum furcatum, &c.). Probably the middle rod is horizontal and serves as supporting base for the central capsule, whilst two opposite spines are directed upwards, two other downwards.

1. Plagonidium bigeminum, n. sp.

Spines straight, three-sided prismatic, four to six times as long as the common central rod, pinnate, with four to five pairs of opposite pinnulæ, the distal of which are simple, the proximal again branched.

Dimensions.—Length of the spines 0.16, of the middle rod 0.032.

Habitat.—Indian Ocean, Sunda Strait (Rabbe), surface.

2. Plagonidium quadrigeminum, n. sp.

Spines cylindrical, curved, eight to ten times as long as the common central rod, in the distal half forked; the fork-branches curved, somewhat longer than the basal part.

Dimensions.—Length of the spines 0.18, of the middle rod 0.02.

Habitat.—Antarctic Ocean, Kerguelen Island, Station 159, surface.

Genus 389. Plagiocarpa,^[6] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus Plagiocarpa agrees with the preceding Plagonidium in the possession of a common horizontal middle rod, the two poles of which bear two pairs of divergent spines; but whilst in the preceding all four spines are equal, here they are differentiated in the same manner as in Plagoniscus, which differs only in the absence of the middle rod. The two observed and closely allied species of this genus are of peculiar interest, since they belong possibly to the common ancestral forms of the Nassellaria; the basal middle rod corresponds perhaps to the basal part of a sagittal ring, the apical spine to its dorsal part, the three other spines to the basal feet (compare above, p. 902).

1. Plagiocarpa procortina, n. sp. (Pl. 91, fig. 5).

Spines cylindrical, curved, thorny, three to four times as long as the common middle rod. Apical spine and the meeting caudal spine (or the posterior basal spine) somewhat longer and with more numerous thorns than the two paired pectoral (or anterior) spines. From the common base of the latter arises an anterior prolongation of the horizontal middle rod, which in the sagittal plane is curved upwards and corresponds to the sternal foot of many Cyrtellaria. An ascending branch of this spine is opposed to a descending branch of the apical spine, both together forming an incomplete sagittal ring.

Dimensions.—Length of the two larger spines 0.15, of the two smaller 0.12.

Habitat.—Mediterranean (Portofino near Genoa), surface.

2. Plagiocarpa procyrtella, n. sp.

Spines of form and arrangement similar to those of the preceding species, but longer and more branched, six to eight times as long as the shorter common middle rod. The two characteristic opposed branches (the ascending branch of the basal rod and the descending branch of the apical spine), which in the preceding species nearly compose a sagittal ring, are here absent.

Dimensions.—Length of the two larger 0.27, of the two smaller 0.21.

Habitat.—North Atlantic, Iceland (Steenstrup), surface.

Subfamily 3. Hexaplagida, Haeckel.

Definition.—Plagonida with six radial spines.

Genus 390. Hexaplagia,^[7] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with six radial spines, arising from one common central point.

The genus Hexaplagia differs from the preceding genera of Plagonida in the possession of six radial spines, diverging from one common central point. Commonly, these six spines seem to lie opposite in pairs in three different meridian planes, and in this case Hexaplagia may be regarded as a Plagiacantha, the three radial spines of which are prolonged over the basal pole of the central capsule. But in other species the six spines seem to lie in different planes. Further observations are required.

1. Hexaplagia arctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, straight, three-sided prismatic, with dentate edges. The three pairs are equal, but the upper spine of each pair only half as long as the lower spine.

Dimensions.—Length of the upper spine 0.13, of the lower 0.27.

Habitat.—Arctic Ocean (Greenland), in the stomach of a Medusa (Olrik).

2. Hexaplagia antarctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, slightly curved, three-sided prismatic, with verticillate branches; the three pairs are equal, but the lower spine of each pair bears three to four verticils, each of three branches, and is two to three times as long as the upper spine, which bears two verticils only.

Dimensions.—Length of the upper spine 0.12, of the lower 0.3.

Habitat.—Antarctic Ocean, Station 157, depth 1950 fathoms.

3. Hexaplagia collaris, n. sp.

Spines in opposite pairs in three diameters, crossed in the common centre, straight, cylindrical, pinnate, with three to four pairs of opposite lateral branches or regular pinnulæ. All six spines are equidistant, of equal size and similar form, and lie with their branches nearly in one horizontal plane. Therefore they are similar to the six radial rods in the collar septum of many Spyroidea and Cyrtoidea (e.g., Calpophæna, Pl. 53, fig. 18).

Dimensions.—Length of all six spines 0.18, of their basal pinnulæ 0.04.

Habitat.—East of New Zealand, Station 169, surface.

4. Hexaplagia australis, n. sp.

Spines unequal, at unequal intervals, not opposite in pairs, cylindrical, irregularly branched and curved. In this species a definite arrangement of the six different spines could not be detected.

Dimensions.—Length of the spines 0.2 to 0.3, of their largest branches 0.1.

Habitat.—South of Australia, Station 160, surface.

Genus 391. Plagonium,^[8] Haeckel, 1881, Prodromus, p. 423.

Definition.—Plagonida with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus Plagonium differs from the preceding closely allied Hexaplagia in the remarkable peculiarity, that the six radial spines do not arise from one common central point, but from the two poles of a common horizontal middle rod; three divergent spines on each pole. The skeleton of Plagonium exhibits therefore the same remarkable form which is found in the isolated spicula of numerous Beloidea (e.g., Lampoxanthium punctatum, Sphærozoum punctatum), and bears the same relation to Hexaplagia that Plagonidium does to Tetraplagia.

1. Plagonium sphærozoum, n. sp. (Pl. 91, fig. 6).

Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, irregularly branched or nearly verticillate, with short thorny branches. Similar to the single spicula of some species of Sphærozoum and Lampoxanthium.

Dimensions.—Length of the spines 0.12, of the middle rod 0.06.

Habitat.—Equatorial Atlantic, Station 347, surface.

2. Plagonium lampoxanthium, n. sp.

Spines irregularly curved, slender, cylindrical, six to eight times as long as the common middle rod, in the proximal half smooth, in the distal half covered with short thorns. (Similar to an isolated spiculum of Lampoxanthium punctatum or of Sphærozoum variabile, Pl. 4, fig. 5.)

Dimensions.—Length of the spines 0.18, of the middle rod 0.03.

Habitat.—North Pacific, Station 240, surface.

3. Plagonium arborescens, n. sp.

Spines irregularly curved and branched, slender, cylindrical, thorny, twelve to sixteen times as long as the middle rod; the branches are large, arborescent, their ramules again ramified and very thorny.

Dimensions.—Length of the spines 0.2 to 0.25, of the middle rod 0.02.

Habitat.—Indian Ocean, Madagascar (Rabbe), surface.

4. Plagonium trigeminum, n. sp.

Spines straight, slender, cylindrical, four to six times as long as the middle rod, in the basal half smooth, in the distal half arborescent, with three to four irregular verticils of ramified branches. (Similar to a single spiculum of Sphærozoum verticillatum, Pl. 4, fig. 7.)

Dimensions.—Length of the spines 0.15 to 0.2, of the middle rod 0.05.

Habitat.—Central Pacific, Station 265, depth 2900 fathoms.

5. Plagonium distriactis, n. sp.

Spines straight and stout, three-sided prismatic, smooth, ten to twelve times as long as the middle rod; each on the distal end cleft into three divergent straight branches, which are again trichotomous.

Dimensions.—Length of the spines 0.15 to 0.18, of the middle rod 0.015.

Habitat.—South Pacific, Station 288, surface.

Subfamily 4. Polyplagida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plagonida with numerous (seven to nine or more) radial spines.

Genus 392. Polyplagia,^[9] n. gen.

Definition.—Plagonida with numerous (seven to nine or more) radial spines, arising from a common centre and lying in different planes.

The genus Polyplagia alone represents the small subfamily of Polyplagida, distinguished from the other Plagonida by the multiplication of the radial spines, the number of which amounts to seven to nine or more. This increased number is commonly the result of an intercalation of new spines between the three or four primary spines; it is sometimes also effected by stronger development of branches of the latter, which become independent. The following five species of this genus are very different, require further investigation, and perhaps represent different genera:—

1. Polyplagia septenaria, n. sp.

Seven radial spines, straight, three-sided prismatic, verticillate, of different sizes; four larger spines correspond to the four axes of a tetrahedron (running from the centre to the four corners), each with five to six verticils of three simple slender branches; one of these four main spines seems to be the apical, the three others the basal spines of Plagiocarpa; in the three meridian planes between the latter and the former lie the three smaller spines, diverging upwards, each with two to three verticils. (Similar to Polyplecta heptacantha, Pl. 91, fig. 12, but without connection between the branches.)

Dimensions.—Length of the four major spines 0.26, of the three minor 0.11.

Habitat.—Central Pacific, Station 274, surface.

2. Polyplagia octonaria, n. sp.

Eight radial spines, straight, three-sided prismatic, of equal size, arising in two opposite groups from the two poles of a short common middle rod (as in Sphærozoum arborescens, Pl. 4, fig. 8, and in other Beloidea). The four spines of each group are divergent, six to eight times as long as the middle rod, each spine armed with three to four verticils of thorny branches.

Dimensions.—Length of the spines 0.15, of the middle rod 0.022.

Habitat.—Indian Ocean, Cocos Islands (Rabbe), surface.

3. Polyplagia novenaria, n. sp.

Nine radial spines of equal size, straight, cylindrical, lying nearly in a horizontal plane, or forming a very flat pyramid. Near the common central point every three spines arise from a short common rod, so that the centre of the skeleton is triradial. Each spine bears towards the apex two divergent straight lateral branches. This species may have been derived from Plagiacantha arachnoides by shortening and reduction of the basal parts of the three original branched primary spines.

Dimensions.—Length of the nine spines 0.24, of the three basal rods 0.02.

Habitat.—North Atlantic, Færöe Channel (Gulf Stream), John Murray, surface.

4. Polyplagia duodenaria, n. sp.

Twelve radial spines of equal size, arising from a common central point, and diverging in different directions. The twelve spines are very large, opposite in six pairs, cylindrical, longitudinally striped (the expression of concentric lamellæ), and with spinulate surface, covered with innumerable small thorns. The basal quarter of each spine is straight and simple, the second quarter twice forked, and these four fork-branches are again in the outer half of the spine richly forked or ramified, with diverging, slightly curved thin branches; each of the twelve spines with about sixty to eighty terminal branches, the ends of which seem to fall into a spherical face. The position of this remarkable species in this family is doubtful.

Dimensions.—Length of the spines 0.12 to 0.16, of the simple basal part 0.04.

Habitat.—South Pacific (off Juan Fernandez), Station 299, surface.

5. Polyplagia viminaria, n. sp.

Numerous (sixteen to twenty or more) radial spines of about equal size, arising from a common central point and diverging in different directions, richly and more or less irregularly branched. The ends of the numerous small branches seem to fall into a spherical face. The large spines of this species have the same form and structure as in the preceding, nearly allied species, but are more numerous and more irregularly branched and disposed.

Dimensions.—Length of the spines 0.2 to 0.25, of the simple basal part 0.05.

Habitat.—North Pacific, Station 241, surface.

Family XLVII. Plectanida, Haeckel.

Plectanida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectoidea with a wattled skeleton, composed of the meeting and united branches of radial spines, which arise from a common central point or central rod, and protect the partly enclosed central capsule.

The family Plectanida comprises those Nassellaria in which the skeleton is composed of radial spines, arising from a common centre, and of a loose wickerwork, produced by concrescence of the meeting branches of those spines. This rudimentary wattled skeleton is either quite irregular or only slightly regular, but it never assumes the form of a complete lattice-shell, as in the Cyrtellaria (the Spyroidea, Botryodea, and Cyrtoidea), nor does it exhibit a ring (as in the Stephoidea). The central capsule is partly or wholly protected, and often entirely enclosed by the wattled skeleton.

Three species only of Plectanida have been hitherto known. The first described form is Plectophora arachnoides, which its discoverer Claparède observed in a living state in 1855 on the western coast of Norway, and considered as a mere variety of his Plagiacantha arachnoides. Two other species were afterwards observed in the Mediterranean, Polyplecta dumetum, 1856, by Johannes Müller (united by him with Acanthodesmia) and Polyplecta polybrocha by myself in 1864. Many new forms are found in the collection of the Challenger, so that we may here distinguish eight genera and twenty-seven species.

The family Plectanida may be divided into four different subfamilies, according to the number of the radial spines which compose the skeleton. Those four subfamilies, as also the genera contained in them, correspond exactly to the four subfamilies of Plagonida, from which they have originated: Triplectida with three, Tetraplectida with four, Hexaplectida with six, and Polyplectida with numerous (seven to nine or more) radial spines. These spines are usually united (as in the corresponding ancestral Plagonida) in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera Periplecta and Plectanium) the spines arise in two opposite groups (each of two or three spines) from the two poles of a common horizontal central rod, which supports the overlying porochora of the central capsule.

The different genera of Plectanida, which are enumerated in the following synopsis, correspond so exactly to the different genera of Plagonida described above, that probably each of the former has arisen from a corresponding genus of the latter. The only difference between the two is, that in all Plagonida the branches and ramules of the radial spines end freely, without growing together, whilst in all Plectanida the meeting ends of the neighbouring spines grow together and so produce a loose and irregular wickerwork. Therefore the latter constantly possess meshes, which are missing in the former. As soon as any form of Plagonida begins to produce meshes by concrescence of meeting branches of the spines, it passes over into a corresponding form of Plectanida. The loose wickerwork or wattlework so produced is sometimes very irregular, at other times more or less regular; but it never assumes the regular form of a complete lattice-shell, as in the Cyrtellaria (Spyroidea, Botryodea, and Cyrtoidea). The general differences which this wickerwork exhibits in the different genera of Plectanida have been already described above (compare pp. 900-904).

The Central Capsule of the Plectanida agrees perfectly with that of the ancestral Plagonida, as well in general form and structure as in the peculiar topographical relation to the radial spines (compare above, p. 905). A slight difference between the two families is effected by the higher development of the skeleton in the Plectanida. Since the branches of the radial spines in this family become united and form a loose wickerwork, the central capsule becomes more or less enclosed by the latter, and more perfectly protected, than in the Plagonida, where the branches remain free. In this respect the Plectanida approach more nearly to the Cyrtoidea with which they are connected by such transitional forms as Plectaniscus and Periplecta on the one hand, Cladoscenium and Pteroscenium on the other.

Synopsis of the Genera of Plectanida.

I. Subfamily Triplectida. Three radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Three spines lying in one horizontal plane,			393. Triplecta.
		Three spines corresponding to the edges of a flat pyramid,			394. Plectophora.
II. Subfamily Tetraplectida Four radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Four spines arising from one common central point.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	395. Tetraplecta.
				One apical spine opposed to three basal spines,	396. Plectaniscus.
		Four spines arising in two pairs from the poles of a common central rod; one apical spine different from three basal spines,			397. Periplecta.
III. Subfamily Hexaplectida. Six radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Six spines arising from one common central point,			398. Hexaplecta.
		Six spines arising in two opposite groups from the poles of a common control rod,			399. Plectanium.
IV. Subfamily Polyplectida. Numerous radial spines.		Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod),			400. Polyplecta.

Subfamily 1. Triplectida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines.

Genus 393. Triplecta,^[10] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus Triplecta is the simplest and most primitive of the Plectanida, and may therefore be regarded as the prototype of this family. The skeleton represents a triangular lattice-plate with three radial beams. On the central union of the latter rests the oral pole of the central capsule. Since the axis of the latter is vertical, the lattice-plate must be horizontal, serving for the expansion of the pseudopodia. Triplecta has arisen from Triplagia by union of the branches of its three radial spines. In the simplest case only three large meshes are formed, corresponding probably to the three cortinar meshes in the collar septum of many Cyrtellaria.

1. Triplecta triangulum, n. sp.

Spines straight, equal, smooth, cylindrical, each in the basal half with one pair of divergent straight lateral branches. The opposed branches of every two neighbouring spines are united by a thin convex bow. In this way arises a very simple, equilateral triangular skeleton, with three equal large meshes.

Dimensions.—Length of the spines 0.14, sides of the triangle 0.16.

Habitat.—Tropical Atlantic, Station 348, depth 2450 fathoms.

2. Triplecta triactis, n. sp. (Pl. 91, fig. 7).

Spines straight, equal, three-sided prismatic, pinnate, each with three to four pairs of opposite slender pinnulæ or lateral branches; the pinnulæ of each spine are correspondingly parallel to the main rods of the other two spines. All pinnulæ branched and connected by a few slender bows, marking the sides of a regular triangle.

Dimensions.—Length of the spines 0.15, sides of the triangle 0.17.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

Genus 394. Plectophora,^[11] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus Plectophora has been derived from Plagiacantha by union of the meeting neighbouring branches of the three radial spines, and exhibits therefore to it the same relation that Triplecta bears to Triplagia. The loose framework so produced corresponds to the three sides of a flat pyramid and supports the central capsule.

1. Plectophora triomma, n. sp.

Spines equal, straight, three-sided prismatic, thorny, connected by three convex bows, the meeting branches of lateral spines arising in one pair from the middle part of each spine. Therefore the skeleton exhibits three large ovate meshes only.

Dimensions.—Length of the spines 0.12, of the meshes 0.08.

Habitat.—North Pacific, Station 238, surface.

2. Plectophora arachnoides, Haeckel.

Plagiacantha arachnoides, var., Claparède, 1858, Études sur les Infusoires et les Rhizopodes, p. 462, Taf. xxii. fig. 9.

Spines straight, cylindrical, trifid, with one pair of lateral branches. The nine diverging, straight, and smooth branches are connected in the distal part by slender concave bows, so that the whole skeleton exhibits nine wide meshes, three larger pentagonal and six smaller triangular.

Dimensions.—Length of the spines 0.14, of their branches 0.07.

Habitat.—North Atlantic, west coast of Norway (Claparède), surface.

3. Plectophora novena, n. sp.

Spines slightly curved, three-sided prismatic, thorny, with two pairs of lateral branches. The six smaller distal branches end freely, whilst the six larger basal branches are connected by slender convex bows. There are therefore nine wide meshes, as in the preceding closely allied species.

Dimensions.—Length of the spines 0.18, of the basal branches 0.08.

Habitat.—North Atlantic, Færöe Channel (Gulf Stream) (John Murray), surface.

4. Plectophora pyramidalis, n. sp.

Spines straight, three-sided prismatic, with three to four verticils of short lateral branches. The branches of the basal verticils are again ramified, and form by connecting bows a delicate loose framework, covering the three sides of a flat pyramid, the three edges of which are the three radial spines.

Dimensions.—Length of the spines 0.2, base of the pyramid 0.16.

Habitat.—Central Pacific, Station 267, surface.

Subfamily 2. Tetraplectida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with four radial spines.

Genus 395. Tetraplecta,^[12] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with four equal radial spines, arising from one common central point and corresponding to the four axes of a tetrahedron.

The genus Tetraplecta has been derived from Tetraplagia by union of the neighbouring branches of the four radial spines, diverging from a common point in different directions. In some forms of this genus the four rods seem to correspond exactly to the four axes, which are directed from the centre of a tetrahedron towards its four corners; whilst in other forms the four rods and the angles between them are perhaps not perfectly equal.

1. Tetraplecta tetrahedra, n. sp.

Spines straight, equal, three-sided prismatic, pinnate, each with three or four pairs of opposite straight slender pinnulæ or lateral branches; the pinnulæ of each side are correspondingly parallel. All pinnulæ connected by a few slender bridges, thus producing a delicate network with irregular rhomboidal meshes.

Dimensions.—Length of the spines 0.15, of the basal branches 0.05.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

2. Tetraplecta quadricornis, n. sp.

Spines in the basal half straight, in the distal half slightly curved and irregularly branched, with two to three pairs of unequal alternate lateral branches, the distal ends of which are connected by a few slender bows, marking the six edges of an irregular tetrahedron.

Dimensions.—Length of the spines 0.22, of the basal branches 0.14.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

3. Tetraplecta pinigera, n. sp. (Pl. 91, fig. 8).

Spines three-sided prismatic, straight, or in the basal half slightly curved, verticillate; each spine with six to eight three-branched regular verticils, tapering gradually towards the distal end. All branches of each spine lie parallel in three equidistant meridian planes, and are connected by delicate parallel threads, perpendicular to the branches. Therefore the skeleton consists of four pine-shaped trees and twelve delicate triangular wings with rectangular meshes.

Dimensions.—Length of the spines 0.25 to 0.3, of the basal branches 0.1 to 0.15.

Habitat.—Central Pacific, Station 271, surface.

Genus 396. Plectaniscus,^[13] nov. gen.

Definition.—Plectanida with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.

The genus Plectaniscus has probably been derived from Plagoniscus by reticular union of the neighbouring branches of the four radial spines, and exhibits therefore to it the same relation that Tetraplecta bears to Tetraplagia. Whilst in these two latter genera the four spines are equal, in the two former genera there is an important difference between a vertical spine (or apical horn) and three divergent (commonly larger) spines, corresponding to the three basal feet of the triradiate Nassellaria. Perhaps, therefore, Plectaniscus is one of the remotest ancestors of the latter; but differs in the absence of a complete lattice-shell.

1. Plectaniscus cortiniscus, n. sp. (Pl. 91, fig. 9).

Spines straight, three-sided prismatic, with three to four verticils of short perpendicular branches; the branches of the distal verticils are simple and free, those of the proximal verticils again ramified and connected by an arachnoidal network of delicate threads. The vertical apical spine (or horn) is scarcely one-third or half as long as the three divergent basal spines (or feet), and the angle between it and the latter is smaller.

Dimensions.—Length of the apical spine 0.05 to 0.1, of the three basal spines 0.18 to 0.2.

Habitat.—North Pacific, Station 244, surface.

2. Plectaniscus tripodiscus, n. sp.

Spines curved, cylindrical, with a variable number of irregular branches, which in the distal half of the spines are free, in the basal half again ramified and connected by a loose spongy framework. The apical spine is shorter and simpler, nearly straight, less ramified.

Dimensions.—Length of the apical spine 0.1, of the three basal spines 0.15 to 0.18.

Habitat.—North Pacific, Station 238, surface.

3. Plectaniscus archiscenium, n. sp.

Spines slightly curved, three-sided prismatic, with two to three verticils of short curved branches. The vertical apical spine is about twice as long as the three basal spines, and connected with them by an irregular loose framework. The three basal spines are connected together only by a simple ring, so that between them remain three large collar holes, like those of the similar and closely allied Archiscenium quadrispinum (Pl. 53, fig. 11).

Dimensions.—Length of the apical spine 0.32, of the three basal spines 0.18.

Habitat.—South Pacific, Station 295, depth 1500 fathoms.

4. Plectaniscus cladoscenium, n. sp.

Spines curved, three-sided prismatic. The three basal spines pinnate, with four to five pairs of opposite curved pinnulæ; the distal pairs are simple and free, the basal pairs branched and connected by an irregular delicate framework, which together with the apical spine forms a spongy cap-shaped shell, similar to Cladoscenium. Apical spine about half as long, simple.

Dimensions.—Length of the apical spine 0.1, of the three basal spines 0.18.

Habitat.—Central Pacific, Station 274, surface.

5. Plectaniscus clathrocorys, n. sp.

Spines straight, three-sided prismatic, with seven or eight verticils of ramified branches, which in the basal half are connected by an irregular loose framework. In the distal half each of the three divergent basal spines (or feet) is connected with the longer apical spine (or horn) by a fenestrated triangular lattice wing. Therefore the skeleton becomes very similar to Clathrocorys (Pl. 64, figs. 8-10), but wants the regular central cephalis.

Dimensions.—Length of the apical spine 0.2, of the basal spines 0.14.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

Genus 397. Periplecta,^[14] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus Periplecta has probably been derived from Plagiocarpa (by concrescence of the meeting spine branches), and has the same important relation to a part of the triradial Stephoidea (Cortina, &c.) and Cyrtoidea (Pteroscenium, &c.). It differs from the latter only in the fact that the loose irregular framework connecting the bases of the four cortinar spines is not a regular lattice-shell.

1. Periplecta cortina, n. sp. (Pl. 91, fig. 10).

Spines of very different size and shape, three-sided prismatic. The apical spine (or horn) is larger, nearly straight, and bears a verticil of three large divergent branches, which are again ramified. The three basal spines (or feet) are curved, pinnate, each with three to five pairs of opposite simple curved branches. A small common central rod separates the two united pectoral feet from the two other spines (the caudal foot and the apical horn). The three basal feet are connected by three convex bows composing a horizontal collar ring, and since the central rod is prolonged between the pectoral feet to the ring, a very remarkable collar septum is formed with four cortinar meshes. Above this septum an irregular spongy shell, including the central capsule, is formed by a delicate framework, interwoven between the apical horn and the three basal feet.

Dimensions.—Length of the apical spine 0.25 to 0.3, of the three basal spines 0.15 to 0.2.

Habitat.—Central Pacific, Station 271, surface.

2. Periplecta pteroscenium, n. sp.

Spines of very different size and shape, three-sided prismatic, verticillate. The apical spine (or horn) with ten to twelve verticils, is one and a half times as long as the three basal spines (or feet), which bear six to eight verticils only; each verticil with three branches which by communicating ramules form a loose irregular wickerwork. Similar to Pteroscenium pinnatum, Pl. 53, figs. 14-16, but without a regular lattice-shell. A short basal central rod separates two equal (pectoral) spines from two unequal spines, the larger of the latter is the apical, the shorter the caudal spine.

Dimensions.—Length of the apical spine 0.26, of the three basal spines 0.17.

Habitat.—Central Pacific, Station 274, surface.

3. Periplecta monocyrtis, n. sp.

Spines of nearly equal size and shape, cylindrical, irregularly branched. The apical spine is straight, more branched and nearly twice as long as the three basal spines. All four spines in the basal third connected by a loose spongy framework, approaching the form of some Monocyrtida.

Dimensions.—Length of the apical spine 0.24, of the basal spines 0.13.

Habitat.—Central Pacific, Station 274, surface.

Subfamily 3. Hexaplectica, Haeckel.

Definition.—Plectanida with six radial spines.

Genus 398. Hexaplecta,^[15] Haeckel, 1881, Prodromus, p. 425.

Definition.—Plectanida with six radial spines, arising from one common central point.

The genus Hexaplecta has probably been derived from Hexaplagia by concrescence of the meeting branches of the six spines, and therefore has to it the same relation that Plectophora bears to Plagiacantha. The two species here described represent perhaps two different genera, since in the first all six spines are equal, opposed in pairs, in the second different in pairs (three upper smaller and three lower larger).

1. Hexaplecta triaxonia, n. sp.

Spines equal, opposite in three crossed pairs, straight, three-sided prismatic, thorny, in the distal half with three leaf-shaped, dentate edges, in the basal half with three verticils of slender branches, which are connected by parallel threads, and so form an arachnoidal network with rectangular or rhomboidal meshes.

Dimensions.—Length of the spines 0.22, of their basal branches 0.06.

Habitat.—South Pacific, Station 285, depth 2375 fathoms.

2. Hexaplecta tricladonia, n. sp.

Spines unequal, three larger spines (in the lower half of the body) being opposite to three smaller spines (in the upper half). The latter are nearly horizontally expanded, each bearing three straight divergent branches. The larger are stronger, twice as long and armed with three verticils, each of three branches. All six spines are slender, three-sided prismatic. Their branches are very thin, thread-shaped, and form by their union a loose network with irregular polygonal meshes.

Dimensions.—Length of the larger spines 0.25, of the smaller 0.12.

Habitat.—South Pacific, Station 291, surface.

Genus 399. Plectanium,^[16] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus Plectanium has been derived from Plagonium by concrescence of the meeting branches, and bears therefore to it the same relation that the preceding Hexaplecta exhibits to Hexaplagia. Whilst in these two latter genera the six spines arise from a common central point, they arise here in two divergent groups from the two poles of a horizontal common middle rod, similar to the spicula of many Beloidea.

1. Plectanium trigeminum, n. sp. (Pl. 91, fig. 11).

Spines straight and stout, six to eight times as long as the common middle rod, three-sided prismatic; in the distal half thickened, with three divergent terminal thorns; in the basal half with three to four verticils of thin lateral branches, which are again ramified, and by their united threads produce an irregular loose framework.

Dimensions.—Length of the spines 0.25, basal breadth 0.005, terminal breadth 0.02; length of the middle rod 0.03.

Habitat.—North Pacific, Station 244, surface.

2. Plectanium ovodimare, n. sp.

Spines straight and stout, three-sided prismatic, about four times as long as the common middle rod, gradually thinned towards the distal end; in the basal half with two to three verticils of forked branches, which are dichotomously ramified, and by their united threads form a loose ovate framework.

Dimensions.—Length of the spines 0.17, of the middle rod 0.04.

Habitat.—North Pacific, Station 236, surface.

3. Plectanium sphærozoum, n. sp.

Spines straight, cylindrical, ten to twelve times as long as the common middle rod, with six to eight verticils of thorny branches, tapering towards the distal end; the branches of the verticils are simple in the distal half, again ramified in the basal half, and here connected together by irregularly branched threads forming a loose framework. (Resembles one spiculum of Sphærozoum verticillatum, Pl. 4, fig. 7, but is more richly branched, with united ramules.)

Dimensions.—Length of the spines 0.2, of the middle rod 0.01.

Habitat.—Central Pacific, Station 266, surface.

Subfamily 4. Polyplectida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with numerous (seven to nine or more) radial spines.

Genus 400. Polyplecta,^[17] Haeckel.

Definition.—Plectanida with numerous (seven to nine or more) radial spines, arising from a common centre and lying in different planes.

The genus Polyplecta comprises provisionally all Plectanida possessing seven or more radial spines, united in the centre of the framework. It may be derived from Polyplagia by meeting and concrescence of the free branches. But as in the latter genus, here also the few observed species are very different, perhaps of different origin, and may be afterwards better separated as representatives of diverse genera.

1. Polyplecta heptacantha, n. sp. (Pl. 91, fig. 12).

Heptaplegma heptacantha, Haeckel, 1882, MS.

Seven unequal spines divergent from one common central point, slender, slightly curved, three-sided prismatic. Four spines are much larger, twice to three times as long and as thick as the three smaller spines. One of the four larger spines is directed upwards (as "apical horn"), whilst the three others diverge downwards (as "basal feet"). Each of the four larger spines bears five to seven equidistant verticils of three divergent branches, the proximal of which are branched, the distal simple. The three smaller spines lie nearly horizontally, midway between the odd apical spine and the three basal spines, and in the same meridian planes with them; each bears one verticil of three divergent branches. All the branches are united by arachnoidal threads, composing a loose wickerwork with irregular, generally quadrangular meshes.

Dimensions.—Length of the four larger spines 0.27 to 0.33, of the four smaller 0.1 to 0.14.

Habitat.—Central Pacific, Station 271, surface.

2. Polyplecta enneacantha, n. sp.

Enneaplegma enneacantha, Haeckel, 1881, Prodromus, p. 425.

Nine equal and equidistant, straight, cylindrical radial spines, lying nearly in one plane, arise from an irregular spongy central framework; six of them seem to be secondary, intercalated between three equidistant primary spines, which are united in the centre. (This species resembles in the nine-radial structure the remarkable Enneaphormis rotula, Pl. 57, fig. 9, and may perhaps be derived from a similar species; but it has no regular latticed shell.)

Dimensions.—Length of the spines 0.22, diameter of the framework 0.17.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

3. Polyplecta decacantha, n. sp.

Pentaplegma decacantha, Haeckel, Prodromus, p. 425.

Ten radial spines, curved, cylindrical, irregularly branched, diverge in different directions and seem to arise in pairs from an irregular central framework, in the centre of which five primary spines are united; the latter correspond probably to the five spines of Pentaspyris, &c. The density of the spongy central framework did not allow of an accurate investigation, and makes it doubtful whether this species is not a Spongiomma.

Dimensions.—Length of the spines 0.3 to 0.4, diameter of the framework 0.18.

Habitat.—Tropical Atlantic, Station 338, depth 1990 fathoms.

4. Polyplecta polybrocha, Haeckel.

? Acanthodesmia polybrocha, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 368, Taf. xxvi. fig. 3. Plegmosphæra polybrocha, Haeckel, 1881, Prodromus, p. 455.

Numerous (twenty to thirty or more) radial spines, thin, cylindrical, curved and irregularly branched, arising from an irregular central spongy framework, are connected by numerous slender arches. The specimen observed by me in 1880 in Portofino was a true Polyplecta, with three primary spines centrally united, between which numerous other spines were intercalated. The similar specimen, however, observed in 1864 in Villafranca, and figured, loc. cit., was perhaps a Plegmosphæra.

Dimensions.—Length of the radial spines 0.05 to 0.1, diameter of the framework 0.16.

Habitat.—Mediterranean (Villafranca, Portofino), surface.

5. Polyplecta dumetum, Haeckel.

Acanthodesmia dumetum, J. Müller, 1868, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 30, Taf. i. Fig. 3.

Numerous (ten to twelve or more) radial spines, thin and straight, with a few straight lateral branches, diverge in different directions and are connected by a few slender curved arches. Some similar forms, but more developed, with numerous branches and curved rods, are found in the Pacific Radiolarian ooze, and represent probably different species.

Dimensions.—Length of the radial spines 0.05 to 0.08, of the branches 0.02 to 0.03.

Habitat.—Mediterranean, French shore (Cette and Saint Tropez), Johannes Müller, surface.

1. Triplagia = Triangular or three-radial; τριπλάγιος.
2. Plagiacantha = Having spines on the sides; πλάγιος, ἄκανθα.
3. Tetraplagia = Four-sided; τετραπλάγιος.
4. Plagoniscus = Side-windlass; πλᾶγος, ὀνίσκος.
5. Plagonidium = Small side-article; πλαγωνίδιον.
6. Plagiocarpa = Having its fruit (the central capsule) on the side; πλαγιόκαρπος.
7. Hexaplagia = Six-sided; ἑξαπλάγιος.
8. Plagonium = Side-article; πλᾶγος, ὤνιον.
9. Polyplagia = Manifold; πολυπλάγιος.
10. Triplecta = Hunting net with three beams; τρίς, πλεκτή.
11. Plectophora = Bearing a hunting net; πλεκτή, φόρος.
12. Tetraplecta = Hunting net with four beams; τέτρα, πλεκτή.
13. Plectaniscus = Shell of wickerwork; πλεκτανίσκος.
14. Periplecta = Surrounded by a hunting net; περί, πλεκτή.
15. Hexaplecta = Hunting net with six beams; ἕξα, πλεκτή.
16. Plectanium = Small wickerwork or network; πλεκτάνη.
17. Polyplecta = Much entwined; πολύπλεκτος.