Natural History Review/Series 2/Volume 1/Number 4/On the Systematic Arrangement of the Rhizopoda

by William Benjamin Carpenter

XLVII.—On the Systematic Arrangement of the Rhizopoda.
By William B. Carpenter, M.D. F.R.S., &c.

Notwithstanding that, by the general consent of zoologists, the group of Rhizopods is now admitted to take rank as a class in the sub-kingdom Protozoa, and although there is little or no difference of opinion as to the extent of range which it comprehends, scarcely anything has yet been done towards the determination of the principles on which its various forms should be classified into Orders and Families; so that among the writings of recent systematists there is a complete disaccordance as to the relative places assigned to them. Having been recently led to inquire into this subject with some care, for the purpose of determining the relations of the Foraminifera to the other members of the class, and having been encouraged to believe that my results may be deemed worthy of acceptance by other Naturalists, I avail myself of the pages of the "Natural History Review" to bring them in a concise form under their consideration; referring to my forthcoming "Introduction to the Study of the Foraminifera;" shortly to be published by the Ray Society, for a fuller exposition of them.

It is not a little singular that Dujardin, who first discovered the true "idea" of the Rhizopodous type,^[1] and to whose original account little of importance has subsequently been added, should have so limited his definition of it as actually to exclude some of what we now regard as its most characteristic examples. In his "Histoire Naturelle des Zoophytes Infusoires" (Paris, 1841), he ranks the Amibiens as the second family of his Infusoires, the Rhizopodes as the third, and the Actinophryens as the fourth; but he distinctly states that the structure of the animal is essentially the same in the first two cases, and that the Rhizopodes are differentiated from the Amibiens solely by the enclosure of their bodies in a testaceous envelope, varying in consistence from a simple flexible membrane to a thick calcareous shell, either solid or porous. He does not, however, regard the differences in the texture of the envelope as equal in importance to those presented by the form of the pseudopodian extensions of the sarcode-body, according to which the Rhizopodes may he divided into two sections; of which the first (corresponding to Ehrenberg's family Arcellina) includes only the Arcellæ and Difflugiæ, whose pseudopodia are short, thick, and rounded at their extremities; whilst the second comprehends all those whose pseudopodia are filiform and much attenuated towards their extremities. This second section was subdivided by Dujardin into three tribes; the first composed of the genera Trinema, Euglypha, and Gromia (all discovered by himself), which are distinguished from Difflugia only by the attenuation of their pseudopodia; the second is composed of the single genus Miliola, which agrees with the ordinary Foraminifera in the possession of a calcareous shell, whilst it corresponds with Gromia in having but a single large aperture from which the pseudopodia extend themselves; and the third includes the Foraminifera proper, all of which were supposed by Dujardin to be furnished (like the few observed by himself) with porous shells for the passage of pseudopodia from the general surface of the body.

Now this arrangement, imperfect though it was, is based (as it seems to me) on a truer perception of the value of characters than most of the classifications that have been since proposed. For Dujardin distinctly recognized the fact that Arcella and Difflugia are nothing else than testaceous Amœbans; and in separating these from those Rhizopods which are characterized by the possession of filiform, tapering, or ramifying pseudopodia, he laid the foundation of a truly natural grouping of the latter. Had he recognized the fact that his group of (testaceous) Rhizopods is related, on the one side, not less closely to Actinophrys than it is, on the other, to Amœba, and thatTrinema and Euglypha are really formed on the Actinophryan type, whilst Gromia is the representative of the Foraminiferous, he would have marked out, upon a sound basis, what appear to me to be the fundamental divisions of the class. Even in separating Miliola from the ordinary Foraminifera, he adopted a principle which I believe to be perfectly correct, though his limited acquaintance with the group misled him in the application of it; for, as I shall hereafter show, Miliola is the type of a large group of Foraminifera in which the body is inclosed by an imperforate shell, so that there is no exit for its pseudopodial extensions except by the apertural plane, in which there is sometimes (as in Miliola) a single large orifice, whilst in other cases it is replaced by a multiplicity of distinct pores. The differentiation between this group and the one in which the shell, being everywhere perforated with pores more or less fine, allows the passage of pseudopodia from every part of the surface of the body, I hold, with Dujardin, to be of essential importance.

These considerations have been altogether passed over, not only by M. D'Orbigny, who adopted Dujardin's rectification of the position of the Foraminifera in the zoological series, without in any way modifying the classification of the group which he had previously devised under the notion that the animals by which these shells are formed are minute Cephalopods, but also by Prof. Schultze, who, having applied himself to the study of the Foraminifera and their allies in the living condition, might be expected to have gained more insight into their true relations as indicated by the characters furnished by their sarcode-bodies. Yet he shows himself to be so completely under the influence of views of systematization based on the characters of the shell, and to have so little regard even to the most important structural and physiological differences anywhere presented by the animals of this class, as to associate in his family Lagynidæ^[2]—for no other reason than that they agree in the possession of a unilocular test, Arcella and Difflugia—whose animals are of the Amœban type, Trinema and Euglypha—whose animals are Actinophryan in character, Gromia—whose animal is the type of that of the imperforate-shelled Foraminifera, Squamulina—which has an imperforate calcareous shell of the Milioline type, and Ovulina—whose shell is perforated. Any arrangement more truly unnatural can scarcely be conceived:—to me it appears a sort of reductio ad absurdum of the principle that the unilocularity or multilocularity of the shell should be held of primary account in the systematic arrangement of the organisms in question.

An important step in the classification of the Rhizopoda was made by the late Prof. Johann Müller, in his admirable memoir (Transactions of the Berlin Academy, 1838), "Uber die Thalaasicollen, Polycystinen, und Acanthometren des Mittelmeeres;"' these three groups, whose mutual affinity he showed to lie very strong, being associated by him into a distinct sub-class, which he distinguished as Rhizopoda Radiolaria. He failed, however, to perceive what appears to me to be the essential relationship between the Acanthometrina and Actinophryna; an Acanthometra, as we shall presently see, being nothing else than an Actinophrys furnished with a siliceous skeleton. And in drawing a strong line of demarcation between the simple and the composite forms of Thalassicolina, he endeavoured to establish a distinction which seems to me untenable among animals that multiply by gemmation, between the simple and the composite forms. Taking the group of Radiolaria as a whole, however, it may be considered an eminently natural one; and I adopt it as one of the primitive sub- divisions of the class, adding to it the family Actinophryna, which includes Actinophrys and its immediate allies, for reasons which will be presently apparent.

More recently an attempt has been made to frame a natural classification of the Rhizopoda as a whole, by two distinguished pupils of Prof. Müller, MM. Claparède and Lachmann ("Études sur les Infusoires et les Rhizopodes," Genève, partie 2ième, 1859); and it is with some diffidence that I venture to express a divergence of opinion from observers who have been trained in so excellent a school, and who have given such ample proofs in their published writings of practical familiarity with the several forms whose relations they discuss. The following is the scheme proposed by them (l.c. p. 434):—

								Orders.		Families.
Rhizopodes.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	No calcareous test No multiple-porous chambers	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Pseudopodia rarely uniting	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	No silicious spicula No yellow cells	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Proteina	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	1. Amœbina.
										2. Actinophryna.
						Silicious spicula Yellow cells	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Echinocystida	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	3. Acanthometrina.
										4. Thalassicollina..
										5. Polycystina.
				Pseudopodia forming very numerous junctions	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	....		Gromida		Gromida
		A usually calcareous test, most frequently multilocular: even when there is but a single chamber, its parietes are traversed by a multitude of pores			$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	....		Foraminifera	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	1. Monothalamia.
										2. Polythalamia.

Now on this I have to remark, in the first place, that the two families Amœbina and Actinophryna, which are associated in the order Proteina, differ essentially from each other in several particulars which seem to me of great; physiological importance; whilst I cannot trace any such peculiar bond of union between them, as would be required to justify their separation from all other Bhizopods and their association into a separate order. Again: the foregoing arrangement follows that of Prof. Müller in dissociating Actinophryna from Acanthometrina, to which they are much more nearly allied than they are to Amœbina. And thirdly, the ordinal separation of Gromida from Foraminifera seems to me to be altogether unwarranted by any essential difference, since the condition of the animal in these two groups is in every respect the same ; while the diversity in the material of the envelopes which they respectively form can no more be admitted as a valid ground of separation in this group than in the family Amœbina, of which Arcella exudes a chitinous test like that of Gromia, whilst Difflugia forms its test by the cementation of foreign particles, as do several genera among Foraminifera.

It is, as it seems to me, in the structural and physiological conditions of the animal alone, that we should look for the characters on which our primary subdivisions should be constituted; and notwithstanding that the extreme simplicity and apparent vagueness of those conditions at first sight appear almost to forbid the attempt to assign to them a differential value, yet a sufficiently careful scrutiny will make it clear that, under their guidance, lines of demarcation may be drawn, as precise as in any other great natural group, between three aggregations of forms which assemble themselves round three well-known types, Amœba, Actinophrys, and Gromia,—the sarcode-bodies of these three types presenting three distinct stages in the differentiation of the protoplasmic substance of which they are composed, and exhibiting, in virtue of that differentiation, three very distinct modes of vital activity.

I.—The lowest stage of this differentiation is seen in Gromia and its allies, among which may be particularly specified a remarkable naked form, which has been described by MM. Claparède and Lachmann under the name of Lieberkühnia, and which seems either identical with the Pamphagus of the late Prof. Bailey (U.S.), or very closely allied to it. In this type the whole substance of the body and of its pseudopodian extensions is composed of a homogeneous, semi-fluid, granular protoplasm, the particles of which, when the animal is in a state of activity, are continually performing a circulatory movement, which has recently been likened by Prof. Schultze (and, as it seems to me, with great justice) to the circulation of the particles in the protoplasmic network within the cell of a Tradescantia. The entire absence of anything like a membranous envelope is evinced by the readiness with which the pseudopodian extensions fuse together whenever they come into contact, and with which the principal branches subdivide into finer and yet finer threads, by whose continual inosculations a network is produced that might be almost described as an animated spider's web. Any small alimentary particles that may come into contact with the glutinous surface of the pseudopodia are retained in adhesion by it, and speedily partake of the general movement going on in their substance. This movement takes place, in two principal directions; from the body towards the extremities of the pseudopodia, and from these extremities back to the body again. In the larger branches a double current may be seen, two streams passing at the same time in opposite directions; but in the liner filaments the current is single, and a granule may be seen to move in one of them to its very extremity, and then to return, perhaps meeting and carrying back with it a granule that was seen advancing in the opposite extremity. Even in the broader processes, granules are sometimes observed to come to a stand, to oscillate for a time, and then to take a retrograde course, as if they had been entangled in the opposing current,—just as is often to be seen in Chara. When a granule arrives at a point where a filament bifurcates, it is often arrested for a time until drawn into one or the other current; and when carried across one of the bridge-like connections into a different band, it not unfrequently meets a current proceeding in the opposite direction, and is thus carried back to the body without having proceeded very- far from it. The pseudopodian network along which this "cyclosis" takes place is continually undergoing changes in its own arrangement; new filaments being put forth in different directions, sometimes from its margin, sometimes from the midst of its ramifications, whilst others are retracted. Not unfrequently it happens, that to a spot where two or more filaments have met, there is an Influx of the protoplasmic substance, which causes it to accumulate there as a sort of secondary centre, from which a new radiation of filamentous processes takes place.

Now, the entire absence of differentiation in the protoplasmic substance, the freedom of the mutual inosculation of its pseudopodian extensions, and the active cyclosis incessantly going on between these and the body, are three mutually related conditions, which not only serve to characterize the group of animals that exhibits them, but, as we shall presently see, to differentiate that group from others. There is, moreover, a negative character of much importance, which is naturally associated with the absence of differentiation,—namely, the deficiency of the "nucleus" and "contractile vesicle" that occur both in Actinophrys and in Amœba. So far as is yet known, there is a perfect agreement as to all these characters between the Foraminifera and the Gromida; and I regard Lieberkühnia as standing in the same relation to the chitine-covered Gromia or to the calcareous-shelled Foraminifera, that Actinophrys does to the chitine-covered Euglypha or to the siliceous-shelled Polycystina. The entire group thus constituted may (as it appears to me) be appropriately termed Rhizopoda Reticularia; the ordinal designation being meant to express that reticulose arrangement of the pseudopodian extensions which is its distinguishing characteristic.

II.—In Actinophrys and its allies there is a degree of definiteness in the form and arrangement of the pseudopodia, which contrasts strongly with the entire indefiniteness which prevails throughout the Reticulose order. These organs are, for the most part, simple filaments, tapering gradually from base to point, usually maintaining their isolation throughout, and extending in a radiary direction from the body of the animal. It is obvious that they are of much firmer consistence than in Gromia and its allies, since they neither subdivide themselves by ramification into finer filaments, nor do they show any readiness to coalesce when they come into mutual contact. Still it is equally certain that they can be retracted into the general mass of the body, and fused (as it were) into its substance; and such a fusion takes place when food is being entrapped by their means. A careful examination of the substance of the Actinophrys serves to explain this apparent inconsistency; for it thence appears that the body and its pseudopodian extensions are far from having the homogeneousness of those of Lieberkühnia, but that there is an incipient differentiation of their substance into two dissimilar constituents, the outer layer being least granular and of firmer consistence, whilst the contained portion approaches more nearly to the character of a liquid, as may be seen by the freer movements of the granular particles which are suspended in it. These two constituents have been appropriately designated by Dr. T. Strethill Wright as the "ectosarc" and the "endosarc." There is no definite line of demarcation between them; but the one graduates insensibly into the others. It seems to be, however, from the ectosarc alone that the pseudopodia are put forth; the granular endosarc not extending itself into them. A movement of granules along their surface may indeed be discerned by careful observation; but these appear to be merely particles which have been entrapped by adhesion to the surface of the pseudopodia, and are being transmitted to the body; and there is nothing like that regular circulation from the body to the extremities of the pseudopodia, and back again, which is so remarkable a feature in the Reticularia. With the incipient differentiation of the protoplasmic substance, there seems to be associated the presence of a "nucleus;" which, however, is not so strongly marked in Actinophrys as it is in Amœba, and may easily escape notice. The "contractile vesicle," on the other hand, is always discernible, and its actions are very regular. Its presence may be considered as superseding the necessity of the general protoplasmic circulation; since it can scarcely be doubted that its function is to maintain a continual movement of nutritive fluid among a system of channels and vacuoles excavated in the substance of the body, some of the vacuoles which are nearest the surface being observed to undergo distention when the vesicle contracts, and to empty themselves gradually as it refills.

The general characters of Actinophrys, with a more or less complete limitation of the pseudopodia to one portion of the body, necessitated by its enclosure within a membranous or chitinous envelope, are presented by the genera Trichodiscus, Plagiophrys, and Euglypha, which are associated with it by MM. Claparède and Lachmann in the family Actinophryna. But they seem to me—so far as I can judge by the published descriptions of these animals, which I have not myself bad the opportunity of examining in their living state^[3]—not less unmistakeably exhibited by the Acanthometrina and the Polycystina, which may be regarded as higher or more specialized forms of the same type. The radiating pseudopodia of Acanthometra correspond precisely in all their characters with those of Actinophrys; having the same rod-like tapering form, the same regularly radiating arrangement, the same mutual isolation, and the same slow movement of particles along their surface: some of them, however, are enclosed in tubular siliceous sheaths, which appear to be secreted from their surface; and the union of the expanded bases of these sheaths forms a sort of framework, that supports the protoplasmic substance of the body. In this substance the differentiation of endosarc and ectosarc has obviously proceeded further than in Actinophrys; and the endosarc contains a number of cell-like bodies resembling those of the Thalassicollina. The animal of the Polycystina seems to correspond with Acanthometra in all essential particulars, the difference being only in the disposition of the siliceous envelope; and that of the Thalassicollina appears to be only a more composite aggregation of the like structural components. For details of the evidence of the relations of the last-named groups to each other and to the preceding, I must refer to the memoir of Prof. Müller already cited; and his designation Radiolaria I adopt as that of the group to which he applied it, with the addition of the family Actinophryna. That family, as I have endeavoured to show, really supplies the typical form of the Order; the naked Actinophrys bearing the same relation to the testaceous Polycystina (for example) that the naked Amœba does to the testaceous Arcella and Difflugia, or the naked Lieberkühnia to the testaceous Gromida and Foraminifera.

III.—From the Actinophryna and the other Rhizopods of the order Radiolaria, the Amœbina seem to me to be very definitely distinguished by the more complete differentiation of the containing and the contained portion of their sarcode-bodies. and by the entire difference (as regards, at least, the typical forms of each group) in the character of their pseudopodial extensions. The distinction between the ectosarc and the endosarc is far more clearly marked in Amœba than in Actinophrys; the latter being much more fluid, whilst the consistence of the former is much firmer. It is through the endosarc alone that those coloured and granular particles are diffused, on which the hue and opacity of the body depend; its central portion seems to have an almost aqueous consistence, the granular particles being seen to move quite freely upon one another, with every change in the shape of the body; but its peripheral portion is more viscid, and graduates insensibly into the firmer substance of the ectosarc. The ectosarc, which is perfectly pellucid, forms an almost membranous investment to the endosarc; still, it is not possessed of such tenacity as to oppose a solution of its continuity at any point, for the introduction of alimentary particles, or for the extrusion of effete matter; and thus there is no evidence, in Amœba and its immediate allies, of the existence of any more definite orifice, either oral or anal, than exists in other Rhizopods. It is asserted by MM. Claparède and Lachmann, however, that an oral orifice does exist in Podostoma, a peculiar modification of the Amœban type; and they think it not impossible that such an aperture may exist even in Amœba, of which the lips might be exactly applied to one another, as in Amphileptus, so as only to open for the ingestion of food. The more advanced differentiation of the ectosarc and the endosarc of Amœba is made evident by the effects of re-agents. If, as Auerbach has shown, an Amœba radiosa be treated with a dilute alkaline solution, the granular and molecular endosarc shrinks together and retreats towards the centre, leaving the radiating extensions of the ectosarc in the condition of cœcal tubes, of which the walls are not soluble, at the ordinary temperature, either in acetic or mineral acids, or in dilute alkaline solutions; thus agreeing with the envelope noticed by Cohn as possessed by Paramecium and other ciliated Infusoria, and with the containing membrane of ordinary animal cells. A nucleus is always distinctly visible in Amœba, adherent to the inner portion of the ectosarc, and projecting from this into the cavity occupied by the endosarc; when most perfectly seen, it presents the aspect of a clear flattened vesicle surrounding a solid and usually spherical nucleolus; it is readily soluble in alkalies and first expands and then dissolves, when treated with acetic or sulphuric acid of moderate strength; but when treated with diluted acids it is rendered darker and more distinct, in consequence of the precipitation of a finely granular substance in the clear vesicular space that surrounds the nucleolus.

In all these particulars, therefore, the Amœbina present a nearer approach to Infusoria than is discernible among other Rhizopods; and hence it was not without good reason that Prof. Müller designated them "Infusorial Rhizopods." They tend towards Infusoria, also, in their higher locomotive powers, obtaining their food by actively going in search for it, instead of entrapping it and drawing it into the substance of their bodies by the agency of their extended pseudopodia. In fact, the pseudopodia are here very different organs from those of either Reticularia or Radiolaria, being rather lobate extensions of the body itself, than appendages proceeding from its surface-layers. They are few in number, short, broad, and rounded; and their outlines present a sharpness which indicates that the substance of which their exterior is composed possesses considerable tenacity. No movement of granules can be seen to take place along the surface of the pseudopodia; and when two of these organs come into contact, they scarcely show any disposition even to mutual cohesion, still less to a fusion of their substance. Sometimes the protrusion seems to be formed by the ectosarc alone, but more commonly the endosarc also passes into it, and an active current of granules maybe seen to pass from what was previously the centre of the body, into the protruded portion, when the latter is undergoing rapid elongation whilst a like current may set towards the centre of the body from some other protrusion which is being withdrawn into it. It is in this manner that an Amœba moves from place to place; a protrusion like the finger of a glove being first formed, into which the substance of the body itself is gradually transferred; and another protrusion being put forth, either in the same or in some different direction, so soon as this transference has been accomplished, or even before it is complete. The kind of progression thus executed by an Amœba is described by most observers as a "rolling" movement, this being certainly the aspect which it commonly seems to present; but it is maintained by MM. Claparède and Lachmann that the appearance of rolling is an optical illusion, for that the nucleus and contractile vesicle always maintain the same position relatively to the rest of the body, and that "creeping," or reptation, would be a truer description of their mode of movement. On this view, these animals have their ventral constantly differentiated from their dorsal surface, it being from the former alone that the pseudopodian extensions proceed; and thus a transition would seem to be indicated towards the testaceous Amœbina (Arcella, Difflugia, &c.) in which the dorsal surface is invested by a shell, and the pseudopodia are strictly limited to the ventral region. It is in the course of its movement from place to place, that the Amœba encounters particles which are fitted to afford it nourishment; and it appears to receive such particles into its interior through any part of the ectosarc, whether of the body itself or of any of its lobose expansions, insoluble particles which resist the digestive process being got rid of in the like primitive fashion.

The Amœban, like the Actinophryan, type shows itself in the testaceous as well as in the naked form; and it is of importance to notice, that whilst the "test" of Arcella is formed by a membranous (probably chitinous) exudation from the animal itself, that of Difflugia is chiefly made up of grains of sand, fragments of shell, or other foreign particles, cemented together. The resemblance of the animals of these two genera is so close, that no systematist has ever proposed to separate them by more than a generic distinction; and if the dissimilarity of the material of their "test" be not admitted as a differential character of grave importance, I can see no reason for attaching more weight to the distinction between the chitinous of Gromia and the calcareous shell of the ordinary Foraminifera, especially as this last is often replaced, either partially or completely, by an envelope formed by the cementation of sandy particles.

Thus, then, Amœba and its allies are distinguished from the Actinophryna, by the yet higher manifestation of that tendency to differentiation of the homogeneous protoplasma, which marks so definite a distinction between the Actinophryna and the Gromida; and the distinction is indicated in the former case, as in the latter, by the nature of the pseudopodian expansions, the lobose form of which seems so characteristic of all the typical Amœbina, that they may be appropriately ranged under the ordinal designation Lobosa. It is quite true that these distinctions do not hold good in every instance; as there are osculant forms (such as the Amœba porrecta of Schultze) whose characters are so intermediate between those of the typical Amœba and of the typical Actinophrys that it is difficult to say to which type they are most nearly allied. And in like manner, judging from the characters of the pseudopodia in Schultze's genera Lagynis and Squamulina, it may be doubted whether the true place of those genera is in association with the Foraminifera, or whether their relation is not really more intimate with the Actino- phryna. But the existence of such osculant forms by no means invalidates the principle of our classification, since their presence only serves to supply, between the Orders into which I propose to divide the Rhizopoda, the link which is necessary to their completeness as natural groups.

It is an. interesting exemplification of the intimacy of the relation between the form of the pseudopodia and the properties of the sarcode-body of the Rhizopoda, that any small separated portion of that body will behave itself after the characteristic fashion of its type; thus, if the shell of an Arcella be crushed, so as to force out a portion of its sarcode, and this be detached from the rest, it will soon begin to put forth lobose extensions like those of an Amœba; whilst if the like operation be performed upon a Polystomella, or any other of the Foraminifera, the detached fragment of the protoplasm will extend itself into delicate ramifying and inosculating pseudopodia, resembling those of Gromia. And this fact seems to me to afford an additional justification of the employment of the characters furnished by the pseudopodia as the basis of a systematic arrangement of the class. The characters of the three Orders into which I propose to distribute its various forms may be concisely summed up as follows:—

I. Reticularia. The body composed of homogeneous granular protoplasm, without any distinction into ectosarc and endosarc; neither nucleus nor contractile vesicle; pseudopodia composed of the same substance as the body, extending and multiplying themselves by minute ramification, and inosculating completely wherever they come into contact; a continual circulation of granular particles throughout the viscid substance of the body and its extensions. This Order consists of the Foraminifera and the Gromida, whose mutual relations will be presently examined.

II. radiolaria. Incipient differentiation of the protoplasmic substance into endosarc and ectosarc, the former semi-fluid and granular, the latter more tenacious and pellucid; a nucleus and contractile vesicle; pseudoppdia rod-like, usually tapering from base to point, composed of the same substance as the ectosarc, exhibiting little disposition either to ramify or to coalesce, haying a more or less regular radiating arrangement, and not showing any constant circulation of granules in their subatance, although a movement of particles adherent to their exterior is often to be distinguished. The type of this order is Actinophrys, constituting, with its immediate allies, the family Actinophryna; but the Order also includes the Acanthometrina, Polycystina, and Thalassicollina, by the last of which this group is connected with the Sponges.

III. Lobosa. More complete differentiation of the protoplasmic substance into endosarc and ectosarc, the former being a slightly viscous granular liquid, and the latter approaching the tenacity of a membrane: a nucleus and contractile vesicle; pseudopodia few and large, being in reality lobose extensions of the body which neither ramify nor coalesce, having well-defined margins, and not exhibiting any movement of granules on their surface, the circulation in their interior being entirely dependent on the changes of form which the body undergoes as a whole. This Order is composed of but a single family, the Amœbina; and it is the one which presents the nearest approximation to the classes Infusoria and Gregarinida.

 

Having thus explained what I conceive to be the true relations of the Foraminifera to other Rhizopods, I purpose now to state the views to which I have been led by the same mode of enquiry, in regard to the classification of that group. And in the first place, it is requisite to examine what is the physiological value of the separation of the Monothalamous, or Unilocular, forms from the Polythalamous, or Multilocular,—a separation which has been hitherto adopted by all systematists as one of primary importance, although Professor Reuss has lately expressed himself doubtfully as to the correctness of its principle.

There can be no doubt that, in common with all the lower forms of animal as well as vegetable life, the Rhizopoda tend to multiply by a separation of continuously-growing parts of their bodies, which may take the form either of fission or of gemmation, according as the original body undergoes subdivision, or as it puts forth an extension which eventually detaches itself. Among the Foraminifera proper, whose bodies are enclosed in unyielding shells, multiplication by fission cannot take place, except in that early stage of existence in which the shell is not yet consolidated; but extension by gemmation may go on without limit, the successively-formed gemmæ usually remaining in connection with each other and with their stock. The progressive growth of the sarcode-substance causes a portion of it to project beyond the aperture of the shell; and this projecting portion possesses all the attributes of the body of which it is an extension, and can maintain its existence with equal readiness, either in a separate state or in continuity with the stock of which it is an offset. Although, therefore, there are certain types of Foraminifera in which such offsets appear invariably to separate themselves before the consolidation of the shell, so that the original body never adds to the number of its segments, and the shell remains "monothalamous,"—whilst there are others in which they ordinarily remain in connection with the original stock, so as progressively to augment the number of the segments and of the chambers of the "polythalamous" shell, often to an indefinite extent,—I cannot see any such difference between the physiological conditions of the newly-formed segment in the two cases, as would be required to justify the erection of the Monothalamia into a distinct order. Moreover, we find that each of these groups, as ordinarily constituted, contains forms which in principle should rank with the other. Thus the continuous spiral shells which are known as Spirillinæ or Cornuspiræ, having their cavities undivided by septa, are always ranked among Monothalamia; but as they have the capacity for indefinite extension, which is characteristic of the Polythalamia, they need nothing but segmental division to turn them into Rotaliæ or Spiroloculinæ. Hence, such shells though actually monothalamous, are potentially polythalamous; and to rank them with Gromiæ, Lagenæ, or Orbulinæ, whose increase can only be effected by the complete detachments of the superfluous segments of sarcode, and by the formation of new and independent envelopes for these,—the enlargement of their shells being forbidden by their shape, would be antagonistic to the very principle on which the differentiation is based. I have recently been investigating another type, not until lately ranked among Foraminifera, which presents a condition of precisely the converse nature. In Dactylopora and Acicularia (as I shall more fully explain in my forthcoming Monograph), we have composite organisms of definite form made up by the aggregation of chambers which have no internal communication with each other, each being as distinct from the rest as the chambers of a heap of Lagenæ, and being only united by external adhesion. Such organisms, therefore, although actually polythalamous, are essentially monothalamous; since the sarcode-body, contained within each chamber, is as independent of the bodies enclosed in the neighbouring chambers, as it would have been if these chambers had been altogether disconnected. Again, there are certain Polythalamia, the successive chambers of whose shells, although formed by continuous gemmation the one from the other, are so slightly connected as to be easily separable by accidental violence, and of which the animals can maintain their lives just as well when they are thus broken up into distinct segments as when retaining their original continuity; such, again, may be regarded as potentially Monothalamous; and the fact that the segments of sarcode, as they were successively budded off from the stock, formed their shelly investments before, instead of after, their detachment from it, can scarcely be admitted by the Physiologist as alone justifying an ordinal differentiation, which is not borne out by other structural or physiological diversities.

Having shown in my former paper how completely fallacious is the assumption of M. D'Orbigny that plan of growth affords the key to the natural arrangement of Foraminifera,—any classification that is founded upon it necessarily bringing together generic types which are physiologically most distinct, and separating such as are physiologically most nearly allied,—I shall now confine myself to a concise exposition of what appear to me the principles on which Natural Classification should be founded.

Looking at the Order Reticularia as a whole, the only great physiological distinction at present known to exist among the multitudinous forms of animal life which it includes (our acquaintance with the mode in which the generative function is performed in this group being as yet so imperfect, that no differential characters can be founded upon it), is that presented by the two modes in which the pseudopodia originate, viz.:—either from the surface of the body generally, or from a limited portion of it. The animals of the former type, of which Rotalia may be taken as an example, have a shell whose surface is everywhere perforated with numerous closely set pores; and through these, as observation shows, the pseudopodia extend themselves freely from each of the segments that occupies the subjacent chambers. In those of the latter, of which Miliola may be taken as the type, the walls of the chambers are entirely imperforated; so that the pseudopodia can only issue from the single or multiple aperture, which leads to the last-formed chamber alone. The fundamental importance of this distinction was perceived (as I have already pointed out) by Dujardin; and my own enquiries, which have been pursued on a basis altogether independent of his, have led me most fully to recognize the merit of that far-sighted perception, which would have been more likely to attract the notice it deserved, if its author had been aware that, instead of being isolated from the true Foraminifera by the characters in question, the Miliolæ are really the representatives of that large group of Foraminifera which are distinguished by the porcellanous texture of their shells.

Taking our stand, then, upon the limitation or diffusion of the origin of the pseudopodia—manifested in the imperforation or the perforation of the testaceous envelope,—as a distinction of fundamental importance, we find that the Order Reticularia may be subdivided by this character into two sections; and as it is convenient to base our systematic arrangement of the Foraminifera upon the characters furnished by the shell (though always hearing in mind that these are of value only in so far as they may be taken as exponents of the characters of the animal) these two sections or sub-orders maybe respectively designated Imperforata and Perforata.

In the sub-order Imperforata, the testaceous envelope presents itself under three very different conditions, the membranous, the porcellanous, and the arenaceous; and upon this difference we may group together the whole aggregate of "imperforate" genera under the three families Gromida, Miliolida, and Lituolida. The family Gromida presents in Lieberkühnia the nearest approach to a naked representative of this Order; the membranous envelope of its sarcode-body being reduced to such extreme tenuity, as only to be distinctly visible where it surrounds the pedicle, from which the pseudopodia are given off; but it is not a little remarkable, and is very significant of the physiological value of the character, that notwithstanding the absence of any shelly wall to limit the extension of the sarcode-body into pseudopodia, these are just as much restricted to one region as if the body had been entirely shut up within an envelope pervious only at one spot. In Gromia, the membranous envelope is of greater firmness, and presents a wide aperture; and the physiological condition of its animal so closely corresponds, except as regards the segmentation of the body, with that of the animal of Miliola, that I cannot see any ground for separating (as M.M. Claparède and Lachmann have done) the Gromida from the Foraminifera proper. Thus I am led to regard Gromia as the unilocular type of the imperforate series; holding the same place in it that Lagena and Orbulina do in the perforated.

The family Miliolida includes an extensive range of generic forms, from the simple undivided Cornuspira (the Spirillina foliacea of Prof. Williamson) to the highly complex and minutely-subdivided Orbitolites. But all these forms are so intimately united with each other, as to constitute an extremely natural assemblage. They all agree in the possession of an imperforate calcareous shell, the substance of which is "porcellanous," being opaque-white by reflected light, and brownish-yellow when sufficiently thin for light to be transmitted through it. The wall of this chamber is simply joined on to that which preceded it, so that the septa between the cavities of adjacent chambers are single, being composed merely of the portions of the walls of the older chambers, which are embraced by the newer. The communications between the successive chambers, and between the last chambers and the exterior (whether formed by a single large aperture as in Miliola, or by the multiplication of smaller pores as in Peneroplis,) are very free; having to give passage not merely to stolons which are subservient to the multiplication of segments, but to bands of sarcode-substance large enough to transmit with facility, to the segments that are furthest removed from the exterior, the nutrient materials obtained by the pseudopodia which issue from the last alone. Neither "intermediate skeleton," nor "canal-system" for its nutrition, presents itself in the Foraminifera of this family; although a sort of representation of it exists in the most complex form of that very aberrant type Dactylopora, which, in addition to the aggregate of separate chambers, has a deposit of solid shell-substance, traversed by a regular system of passages that has no communication with the chambers, but seems to have been in connection with a sarcode-body outside of them.

We occasionally find among the Miliolida that the surface of the shell is formed of arenaceous particles; but these are embedded in a cement formed of the proper shell-substance, which is never wanting; and the close accordance in every other character between shells which are thus superficially altered and such us conform to the ordinary type, forbids our regarding the former as more than varietally distinct from the latter. The case is very different, however, with regard to certain genera in which the power of forming a proper shell seems to be altogether wanting; the testaceous envelope being essentially composed of substance directly derived from without, the only material furnished by the animal being the organic glue that holds them together, their substance is generally composed of a very fine cement in which coarser particles are imbedded; the former Sometimes predominating, so that the shell is smoothed off on the surface; whilst if the latter be in excess, the surface of the shell is rough. Of this family the genus Lituola is the most characteristic; and the variety of forms into which it passes, several of them so closely resembling those of other genera as to have been mistaken for them, would not be readily conceived by any but such as have made a special study of them.^[4] In the genus Trochammina (Parker and Rupert Jones) We have an instance of a gradational transition from the monothalamous to the polythalamous type; for whilst its lowest form is a continuous vermicular spiral (the Spirillina arenacea of Prof. Williamson), this comes to present, in some instances, a degree of segmental division scarcely inferior to that which some of the most vermiculate forms of Rotalia are reduced. The genus Valvulina forms the transition between this group and the "perforated" series; for whilst the principal part of its "test" is uniformly made up of an aggregation of sandy particles, leaving no such pores for the exit of pseudopodia, as can be readily discerned in the arenaceous Textulariæ, this has a basis of true shell-substance in which pores can be distinguished.

In the whole of the sub-order Perforata, the shell is calcareous, and is formed of a dense hyaline or vitreous substance, which is traversed by tubuli running straight from the cavity of the chambers to the external surface, whose diameter usually ranges from 1-3000th of an inch (as in Rotalia and Planorbulina) to less than 1-10,000th (as in Operculina and Cycloclypeus). There can be no question that even the smallest of these tubuli are large enough to transmit the finest threads into which the protoplasmic substance may sub-divide itself: and looking to their remarkable continuity through successive layers of shell substance, when (as in Operculina) the earlier whorls are completely embraced by the later, there can, I think, he no reasonable doubt that, through their means, a direct communication is maintained between even the earliest and innermost segments and the surrounding medium.^[5] This, of course, renders the successive segments much more independent of one another, than they are in the porcellanous type; and their isolation is marked by these two important peculiarities in the structure of the shell,—first, that each segment has its own complete wall, so that the septa between successive chambers are double,—and second, that the apertures of communication through the septa are far smaller than in porcellanous shells, as is seen in comparing a Vertebralina or Miliola with a Nodosaria or Cristellaria, or, in the unilocular types, on comparing the aperture of a Gromia with that of a Lagena. It is in this type alone that we meet with an "intermediate skeleton" nourished by a "canal system" that is connected with the cavities of the chambers; although this feature is wanting in the lower types of the series, yet its presence in the higher, most strongly differentiates them from the forms of the porcellanous type to which they bear the closest resemblance. In certain genera of this as of the porcellanous series, we find the surface of the shell occasionally roughened by the adhesion of arenaceous particles; but these are imbedded in true shell-substance, which is never wanting; and as the very same forms may be altogether free from arenaceous deposit, its presence is obviously not essential but is (so to speak) accidental, and constitutes no ground for even specific distinction.

As the texture of the shell throughout the whole of this series is essentially the same,—the variation in the diameter of its tubuli being the only difference of any mark,—we have not the same easy means of subdividing the Perforated group into families as we possess in the case of the Imperforate; and this division must consequently be based on the aggregate of characters supplied by the coarseness or firmness of the tubuli, the mode of communication between the chambers, and the general plan of growth. To enter into details upon these points would be foreign to my present purpose, which has been merely to set forth the general results at which I have arrived; and these I now offer to the criticism of such Naturalists as interest themselves in the study of the group to which they relate.

1. "Observations sur les Rhizopodes et les Infusoires;" in Comptes Rendus, 1835, p. 338.
2. See his treatise, "Ueber den Organismus der Polythalamien (Foraminiferen) nebst Bermerkungen über die Rhizopoden in allgemeinen." Leipzig, 1854.
3. I am not aware that Acanthometræ have yet been seen upon our coasts. They seem, however, to abound in the North Sea, and should therefore, he looked for upon our eastern shores, especially when the wind blows towards them. The Acanthometra echineides, which abounds on the western coast of Norway, is discernible by the naked eyes a crimson red point.
4. My knowledge of these arenaceous types has been entirely obtained through the kindness of Messrs. Parker and Rupert Jones
5. The finely tubular shell-substance of Operculina and its allies presents a very striking resemblance to dentine in everything except the ramification of the tubuli; and it comes to be a very interesting inquiry what relation there may be between these two substances as to the mode of their formation. There is reason to consider the shell-substance of the Foraminifera as an excretion from the protoplasmic mass of which the body itself is composed; just as the cellulose wall of the vegetable cell, which may be consolidated by carbonate of lime (as in Corallines) or by silex (as in Diatoms) is an excretion from the contained endochrome. The new lamellæ of shell successively added to the external surface of the preceding, in cases in which the spiral lamina of each new whorl completely invests the old, would block up its pores, if the continuity of the tubuli were not maintained by the extension of the pseudopodia through the freshly consolidating substance, and this, by moulding itself upon the pseudopodia that issue from the orifices of the subjacent surface, will itself be rendered tubular, and will continue to allow the passage of the pseudopodia from the earliest chambers through the last formed layer of shell. And I would suggest it as a subject for inquiry whether in the formation of dentine and other calcified tubular tissues of higher animals, the tubular structure is not really the result of the consolidation of an excretion-substance around filamentous prolongations of the active protoplasmic substratum from which it is exuded.