1911 Encyclopædia Britannica/Planarians

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PLANARIANS, a well-defined group of animals, characterized externally by their ovoid or vermiform shape, their gliding movement and their soft, unsegmented, ciliated bodies: internally by that combination of low somatic type of structure and complex gonidial organization which is characteristic of the Platyelmia (q.v.). Their low type of bodily structure may be exemplified by the facts that the mouth is the only means of ingress to and egress from the blind alimentary sac, and that no vascular system is differentiated. Most Planarians are aquatic and the cilia that cover the body produce by their beating a stirring of the water. Hence the class is generally known by the name Turbellaria.

Planarians form one of the basal groups of the animal kingdom. They are the simplest of multicellular creeping things. In them the gliding movement has become habitual. The lowest Planarians are still largely free-swimming animalculæ and we can trace within the limits of the group the development of the creeping habit and the consequences that flow from it. It has led to the differentiation of anterior and posterior extremities, to the formation of bilateral symmetry; and to the development of a mucilage protecting the body against friction. It entails the concentration of the scattered nervous system on the ventral surface and at the anterior end, and it has induced the segregation of the diffused sense-organs in the head. The Planarians occupy a position midway between the simple planula larva of Coelenterates and the segmented Annelids. They have probably sprung either from an early Coelomate stock, or represent an independent class descended from a two layered parentage distinct from that of the Coelenterates; a view which is adopted in the present article.

Occurrence.—Most Turbellaria are aquatic. They abound on the seashore and in fresh water, amongst weeds or under cover of stones, shells and sand. Few of them are pelagic or deep-water forms, and only some half-dozen Planarians are known to be parasitic. A large number of land Planarians are known, chiefly from tropical and south temperate countries.

The majority of marine Planarians are nocturnal or cryptozoic, hiding away during the period of low tide to avoid desiccation of their soft sticky bodies and coming out at night or during high tide to feed. They are mostly carnivorous, and their movements are correlated largely with the nature of their food. The smaller, more active species occur in companies amongst the finer seaweeds over which they creep or swim in pursuit of their food. The larger marine species occur singly or in pairs on Ascidians, Nullipores or Polyzoa, from whence as the tide rises they issue to feed. By the time the next low tide exposes them, these Planarians have so completely digested their meal that we know very little of its nature. The common fresh-water Planarians form either little companies of a dozen or more, usually of a single species, huddled together under a stone or in some cranny (see Pearl [8][1]), or societies of several species that inhabit Sphagnum and other fresh-water vegetation. This fresh-water planarian fauna is of two kinds, the fauna of permanent and that of temporary sheets of water and both show a certain adaptation to their environment. The latter, being subject to greater extremes of temperature than the lacustrine Planarians, produce thick-shelled eggs only. The development of these eggs is rapid in warm water, slow in cold: so that a pool after a few days of early spring sunshine is soon populated and provision is made for the continuance of the race should a cold snap follow. The lacustrine Planarians exhibit a different form of adaptation. The eggs laid by many of these animals are either thin-shelled and rapidly hatched or thick-shelled and slowly hatched. The lake-water, however, is in spring, even after sunshine, of a much lower temperature than that of pool-water, but the masses of Sphagnum and other weeds that border lakes and marshes are often warmer than the open water and may be as much as 13° or 15° C. higher in temperature. Here the Planarians assemble to benefit by the warmth, and under such favourable conditions lay thin-shelled eggs which rapidly develop; whilst in colder surroundings or at the onset of winter thick-shelled resting eggs are laid. In this manner we can understand the abundance of Planarian life in cold meres and transitory pools in Great Britain, Scandinavia, Finland, Denmark and North America.

In contrast to the general habit among Turbellaria of haunting dim or dark places, the station chosen by a few species is exposed and strongly illuminated. The marine Convoluta and Polychaerus and the fresh-water Vortex viridis may be taken as examples. Convoluta paradosa occurs among brown weeds which receive much light during neap tides and strong direct sun or light every fortnight. Polychaerus creeps about the New England shore without resorting habitually to cover, and is also strongly insulated. Vortex resembles the green Hydra of our ponds in choosing the lightest side of its surroundings; and finally, Convoluta roscoffensis paints the beach green in Brittany, part of Normandy and Natal. In every such case the Planarian is coloured brown or green by the presence of photosynthetically active cells and the singular heliotropic habit of these Turbellaria is associated with the illumination necessary for the activity of their coloured cells.

Only one branch of the Planarians has become terrestrial, but this has spread over almost all the whole globe. One species (Rhynchodemus terrestris, fig. 1, e) is fairly common in Great Britain under stones, logs and occasionally on fungi, but the Holarctic countries (North America, Europe and North Africa, North Asia) are extremely poor in terrestrial species. In countries lying in the centre and in the south of the great continents and in the south temperate continental islands and archipelagoes these land Planarians become more abundant and varied; and being frequently transported with earth or plants they are often found in hothouses and botanical gardens far from their native country. Their distribution offers some points of special interest showing a close relationship between the South American fauna and that of Australia and New Zealand: between the land Planarians of Madagascar, of Ceylon and of Indo-Malaya: and a marked contrast between Japan and the rest of the Palaearctic region (see Von Graff [1], 1899).

External Characters.—-Planarians range from the minute forms no larger than Infusoria to ovate, marine species, 6 in. in diameter and to ribbon-like land forms 8 in. in length. The majority are small, somewhat cylindrical organisms with a flat creeping surface. Others, comprising the common freshwater and marine forms, are flattened and leaf-like, often provided with a pair of tentacles near the front end of the body, and in some cases the whole dorsal surface is beset with papillae. The land forms are elongate and smooth, and their anterior extremity is often modified into the arcuate shape of a cheese-cutter. Their movements are usually of a gliding character. The minuter forms perform short excursions into the water round their station, and in so doing recall Infusoria. The larger forms, in addition to gliding like pellicles, fold the expanded anterior part of their body into a couple of fins, with which they swim after the fashion of a skate. The folded margins of other forms clasp the weeds on which they live. Adhesion is effected by the mucous investment of the body and frequently by some specially developed local secretion of slime, or by a sucker. By these means, aided by their algal-frequenting and cryptic habits, the Turbellaria, though soft-bodied, are able to withstand the violence of the waves.

EB1911 Planarians - examples.jpg
Fig. 1.
a Convoluta paradoxa, Oe.
b, Vortex viridis, M. Sch.
c, Monotus fuscus, Gff.
d, Thysanozoon brochii, Gr., with elevated anterior extremity (after Joh. Schmidt).
e, Rhynchodemus terrestris, O. F. Müller (after Kennel).
f, Bipalium ceres, Mos. (after Moseley).
g, Polycelis cornuta, O. Sch., attached by the pharynx (ph) to a dead worm (after Johnson).

All the figures of natural size, and viewed from the dorsal surface. a, c and d are marine, b and g are fresh-water, e and f are terrestrial. All found in Great Britain except d.

The anterior end in all Turbellaria is the site of the chief sense-organs, and in some forms (Proboscida) becomes transformed into an invaginable proboscis of highly tactile nature. Such forms lead naturally to the Nemertina (q.v.).

Coloration.—The coloration of Planarians is of interest. The flattened marine forms are often brilliantly coloured on the dorsal surface, either uniformly or with some striking marginal band; or they may exhibit longitudinal bands of contrasting tints or a mottled appearance. The significance of these colours is not fully understood, but in some cases of sympathetic coloration the derivative function of the pigments is probably to aid cryptic resemblance. The terrestrial Planarians exhibit the most striking patterns in longitudinal striping and cross-bars which appear to have no relation to the environment of these essentially nocturnal animals. The fresh-water forms are colourless or dusky, often dark-brown, possibly in relation to the retention of heat, but in a number of both fresh-water and marine Planarians a green colour is present, constantly in some species, sporadically in others.

This green effect is due to the infection of the Planarian by a minute alga which multiplies in the tissues and may profoundly affect the habits and even the structure of its “host.” The planarian so affected acquires a heliotropic habit; it becomes gregarious and in extreme cases ceases to ingest solid food. In Convoluta roscoffensis the green cells have become indispensable. They function both as the nutritive and excretory organs of the Planarian, and the young animal cannot develop until it is infected and has acquired a supply of these green cells which become incorporated into its tissues (Gamble and Keeble [7]). Brown algal cells (Zooxanthellae) are known in other species of Convoluta.

Food.—The food of Turbellarians consists, in the smaller species, of diatoms, unicellular algae, microscopic animals and other Turbellarians; in the larger ones, of worms, mollusca and insects. The fine feeders capture their food chiefly at night by gulping down the minute organisms that settle or swim in their neighbourhood. The coarse feeders enclose their prey with a coating of slime and then proceed either to engulf it in their expansible mouth or to perforate it by their trumpet-like pharynx. The mouth is remarkably variable in position (fig. 2). In many flattened Planarians it is placed centrally on the ventral surface somewhat as in a jelly-fish. In the majority it is nearer the anterior end, but in a few remarkably elongate forms it occupies a position near the hinder end of the animal. In the cylindrical forms (Rhabdocoels) a similar variability in the position of the mouth is met with.

Anatomy.—The structure of the Turbellaria though greatly varied in detail, conforms to a single type of somatic organization which is transitory in the higher invertebrates. The sexual organs, on the other hand, are founded on two or more types, and the astounding complications of these structures suggest that their evolution has been governed by quite other factors or combinations of factors than those that have guided the somatic evolution of the group.

EB1911 Planarians - group of Polyclad Turbellaria.jpg

(From Cambridge Natural History, vol. ii. “Worms, &c.,” by permission of Macmillan & Co., Ltd. After Lang.)

Fig. 2.—A group of Polyclad Turbellaria, illustrating the various positions in which the mouth of Planarians may occur, and the concomitant changes in other organs.

A,  Anonymus virilis: mouth central, male genital aperture (♂) multiple and biradial.
B, Prosthiostomun siphunculus: mouth anterior, the pharynx protruded through it.
C, Cestoplana: mouth posterior (m); ♂, male; ♀, female genital aperture; Br. brain; CG, eyes especially related to the brain; Ey, marginal eyes; m, mouth; MG, stomach; Ph, Pharynx; s, sucker.

EB1911 Planarians - structure of the simplest Turbellaria.jpg
(After Böhmig.)
Fig. 3.—To show the structure of the simplest Turbellaria.

The figure represents the left half of a transverse section across the body of the Acoelous planarian Halplodiscus. The mouth (M) is plugged up with a digestive polynuclear mass of cytoplasm and the transitions from this to the stellate scattered central parenchyma (SC) and again from the latter to a firmer peripteral zone (PC) are shown. The outermost layer (EP) is a ciliate epidermis resting on (BM), a basement membrane (dark line), the row of dots beneath this represents the longitudinal muscles (L).

The general structural characters are as follows. The body consists of a muscular envelope covered externally by a ciliated glandular epidermis and of an alimentary sac, cylindrical or branched, for which the mouth serves both as ingress and egress. Between this aproctous gut and the integument the body consists of a jelly-like, vacuolated mesenchyme made up of branched gland-cells, excretory cells, pigment- and muscle-cells. A space may be secondarily hollowed out around part of the gut; but no coelomic or true perivisceral cavity exists in the sense in which these terms are used in higher animals. A nervous system is present and consists of an anterior “brain” and of ramifying ganglionic trunks that are developed in relation to the muscular integument and to the sense-organs for the perception of light and pressure. No respiratory organs are developed, probably in correlation with the absence of a blood-vascular system. On the other hand, the process of reproduction is elaborately organized. The Planarians are hermaphrodite and, as in so many other small animals, the body, after attaining maturity, becomes in many Planarians practically a genital sac and is soon exhausted by the repeated calls upon its reserves that are involved in the rapid production of eggs and spermatozoa. The intervals between successive clutches has been found in Convoluta roscoffensis to be a month, thus suggesting the influence of the lunar tides upon maturation.

EB1911 Planarians - transverse section of Mesostoma ehrenbergii.jpg

(Partly after Luther: Zeitschrift für wissenschaft. Zoologie, by permission of Wilhelm Engelmann.)

Fig. 4.—Portion of a transverse section of Mesostoma ehrenbergii.

The epidermis (E) consists of cells divided into an outer and inner zone, the latter containing rhabdites (Rb); the cilia (Ci) are thickened about the middle of their length. Below the epidermis is the basement-membrane (BM), a layer of circular muscles (C) and of longitudinal ones (L). Below this again is the mesenchyma (M), made up of branched cells and dorso-ventral muscle-fibres (DM). The mesenchymatous glands (Rmc) are producing rhammites (Rm) which pass outwards.

Integument.—The epidermis is ciliated and highly glandular. It consists of a single layer of cubical or oblong cells with the structure seen in fig. 3. The glandular secretion takes various forms, such as mucus, mucinoid granular blocks, or fusiform refringent homogeneous rods. These rods or “rhabdites” are frequently coloured red or yellow, and are highly characteristic of the Turbellaria. Their rea use is unknown. In only two genera does the epidermis produce cuticular spines (Acanthozoon, Enantia) on the surface, but chitinoid hooks, spines and spirals occur frequently on the lining membrane of the male and female copulatory ducts.

Below the epidermis is a firm basement membrane into which the subjacent muscles are inserted. They are divided into outer circular and inner longitudinal groups and subdivided in the larger forms by diagonal fibres, and in the most highly differentiated Planarians there are six muscular layers, two of each kind. In a number of Turbellaria the musculature is modified to form a sucker either single or double and anterior or posterior, and it undergoes further modification in connexion with the pharynx and reproductive organs.

EB1911 Planarians - Integument of Mesostoma lingua.jpg
Fig. 5.—Integument of Mesostoma lingua, O. Sch.

On the right hand is the epidermis (z) with perforations (l) through which the rhabdites (st) project. Beneath this the basement membrane (bm), and beneath this again the muscular layers consisting of circular (rm), diagonal (sm), and longitudinal (lm) fibres.

Alimentary Sac.—The alimentary sac consists of a muscular pharynx opening outwards through the mouth and inwards into a median digestive organ which may be solid or hollow, and in the latter case straight, lobate or branched. These characters are correlated with such a number of distinctive features that the classification of the Planarian is based on them. Thus we have the Rhabdocoelida with straight gut and the Tricladida and the Polycladida with triple and multiple branches to the gut. The Rhabdocoelida are further divided into three groups, the Acoela with a simple syncytial gut not sharply separated from the surrounding mesenchyma; the Rhabdocoela, with a hollow gut and a perivisceral schizocoelic span; and the Alloeocoela with a lobate gut and reduced schizocoele. The last group leads one naturally to the Tricladida; the Polyclads being an independent group.

EB1911 Planarians - Excretory System of Mesostoma ehrenbergii.jpg

Fig. 6.—Main trunks of the Excretory System of Mesostoma ehrenbergii, O. Sch., opening to the exterior through the mouth; ph, Pharynx.

EB1911 Planarians - Flame-cell from the Excretory System of Mesostoma ehrenbergii.jpg

(From Lankester's Treatise on Zoology, Part IV.)

Fig. 7.—Flame-cell from the Excretory System.

a, nucleus; b, excretory granules; c, “flame”; d, branches of cell; e, beginning of excretory tube.

EB1911 Planarians - Nervous System of Haplodiscus.jpg

Fig. 8.—The Nervous System of a Simple Planarian (Haplodiscus, one of the Acoela).

B, the brain which gives off a dorsal (DN) and a ventral (VN) plexus and also lateral nerves (LN). The mouth (M) and the otocyst (OT) are shown. The former is ventral, the latter dorsal in position.

The pharynx varies widely in structure. In the Acoela it is a mere thickening and pitting of the integument. In the Rhabdocoela a great number of elaborate modifications are found. These are based on the type of a buccal invagination, which forms the pharyngeal sheath, and from the bottom of this there springs a muscular outwardly directed tube or fold. In the Alloeocoela and Tricladida the pharynx is an elongate protrusible cylinder, and in the Polyclads it may be an immensely distensible frilled organ, the folds of which have independent movement, or an elongate tube. At the base of the pharynx lie the openings of salivary glands. In the Polycladida the section of the alimentary sac into which the pharynx opens is a median stomach from which the intestinal branches radiate. The stomach in few forms is provided with digestive glands. The branches possess an independent musculature and exhibit active peristalsis. The intestine of Planarians is not ciliated, and digestion appears to be largely intracellular and not cavitary.

Mesenchyma.The mesenchyma (Böhmig: parenchyma auctt.) consists of a mass of branched vacuolated cells, imbedded in which lie gland-cells, pigment-cells and the excretory system. It envelops the genital organs, which though in the mesenchyma are not of it, and it forms an investment to the gut and to the space (schizocoel) which often occurs between the gut and the mass of the mesenchyma. The mesenchymatous gland-cells are of different kinds. (1) Single cells in which rods (rhammites) are developed (fig. 4, Rmc). Such cells in embryonic life give rise to a process which perforates the soft basement-membrane and penetrates between the epidermal cells. The process becomes hollow, and the rhammites pass outwards along it on to the surface of the animal, forming in many Turbellarians thickly set rows of rods on the head. (2) Similar cells contain nematocysts in a few Planarians (Microstoma, Stenostoma, Anonymus virilis and Stylochoplana tarda). Whether these nematocysts develop in the Turbellarian is doubtful, and it is not impossible that they are derived from the tissues of some coelenterate animals eaten by the Planarian, as has been shown to be the case in the nematocysts of Eolids. (3) Cells producing aciculate spicules, sometimes associated with a spiral thread. These structures are often associated together in batteries, notably so in the remarkable genus Anonymus.

EB1911 Planarians - Double Eye found on the brain of Polyclads.jpg

(From Cambridge Natural History, vol ii. “Worms, &c.,” by permission of Macmillan & Co., Ltd. After Lang.)

Fig. 9.—Double Eye found on the brain of Polyclads. Each consists of a pigment-cup and of four nerve-end cells (rod-cells) in which the nerves terminate.

N, nerve fibres and cells. Pc, pigment-cell. Rc, rod cell.
EB1911 Planarians - Microstoma lineare undergoing division.jpg

Fig. 10.—Microstoma lineare, Oe., undergoing division. There are 16 individuals. 8 with mouth apertures, showing the buds of the first (m), second (m′), third (m′′), and fourth (m′′′) generation. The fifth generation has not yet acquired a mouth aperture. c, ciliated grooves; e, eye spots; i, intestine.

Excretory System.—The excretory system consists of protonephridia, that is, of tubes opening to the exterior by one or more apertures, and after branching extensively in the mesenchyma, end blindly in peculiar hollow cells (flame-cells) provided with a bunch of synchronously vibrating cilia. The excretory tubules have a markedly sinuous course and are provided with cilia. The motion of these cilia and of the flame-cells is to induce an outward current of the fluid from the canals, but the process of excretion seems to be performed chiefly by the branched mesenchymatous flame-cells. The position of the external opening varies greatly. It may be single or paired, mid ventral or terminal, or again multiple and arranged in pairs along the dorsal surface (Tricladida and probably in Polycladida). The flame-cells are arranged in pairs in Tricladida, but lie less regularly in the mesenchyma of most forms. Finally, it is noteworthy that in the Acoela no excretory system is known.

Nervous System.—The nervous system is present in all divisions of the order. It consists of a paired, anterior ganglion lying ventral to the gut, and from this are given off, right and left, dorsal, lateral and ventral fibres interconnected by a plexus. The nerve-cells are scattered throughout the plexus. The chief development of the system occurs in relation to the muscular body-wall, sense-organs and the pharynx. In these characters the nervous system of Planarians shows an interesting transition from the scattered plexus of Coelenterates to the segmental ganglia and sympathetic nervous system of Annelids.

Sense-organs.—These occur in the form of tactile organs, otocysts and eyes. The whole skin of many Planarians is sensitive, and amongst the ordinary locomotor cilia long stiff ones are found which it is natural to think are tactile organs. The head-end is often provided with specialized cells that appear to subserve the sense of touch and possibly of taste also. The abundance of rhammites, of long stiff cilia, and the great mobility and sensitiveness of this region, bear out this conclusion. A further development of cephalic, sensory structures occurs in the form of a crescentic groove (Polyclads) of paired, lateral pits (Microstoma, fig. 10) of mobile papillae on the extreme front margin (Land Triclads) and of extensible tentacles, marginal or nuchal in most Polyclads.

The otocyst occurs constantly in the Acoela and sporadically in every other division of the group. It is with one exception a single median organ placed over the brain, and consists of a uni- or bi-cellular sac containing a calcareous concretion lying in a fluid. From what is known of these organs in higher invertebrates we may infer that they serve to increase the perception of slow wave-movement and enhance the control of the muscular sense.

Eyes are generally present in Planarians. Two types are distinguishable—eyes with a cup-shaped retina facing outwards, and those with an inverted retina facing inwards. The former occur in Triclads and Polyclads around the margin of the body often a hundred or more may be present. The latter occur in all groups except the Acoela, but are limited to the neighbourhood of the brain or bases of the nuchal tentacles. Recent investigation has shown that the essential part of the eyes has in all cases a complicated structure and is not a mere epidermal cell-group enclosed by pigment and provided with an optic nerve. On the contrary (Hesse [10]), adequately known eyes are composed of rod-cells that contain each an axial filament or bundle of fibrillae (the termination of the nerve), and the distal end of the rod-cell is converted into a striated usually broad border where the action of light commences. A group of such specialized rod-cells is enclosed in a pigmented cup opening either outwards or inwards and pierced by an optic nerve. The whole is usually depressed beneath the epidermis, but in some Acoela and Alloeocoela the eyes retain a surface-position. In the Polyclads eyes may increase by division and in Triclads may decrease in number by fusion (Carrière [11]). The marginal and often radial disposition of the scattered eyes, and the prostomial position of the paired eyes, afford interesting evidence of the intermediate position that Planarians occupy between the radiate Coelentera and the bilateral Annelids.

EB1911 Planarians - Plan of an Acoelous Turbellarian.jpg

Fig. 11.—Plan of an Acoelous Turbellarian.

e, Eye.
m, Mucous gland, formerly mistaken for the mouth, which lies in the centre of the body.
ot, Otolith.
ov Ovary.
p, Digesting parenchyma.
t, Testicular follicles.
vs, Vesicula seminalis.
♂, Male-organ of copulation.
♂♀,  Common sexual aperture.
EB1911 Planarians - Plan of an Rhabdocoelous Turbellarian.jpg

Fig. 12.—Plan of a Rhabdocoelous Turbellarian.

bc Bursa copulatrix.
cn, Brain.
e, Eye.
g, Germarium.
i, Intestine.
ln, Longitudinal nerve trunk.
m, Mouth.
ph, Pharynx.
rs, Receptaculum seminis.
s, Salivary gland.
t, Testis.
u, Uterus (containing an egg).
v, Yelk gland.
vs, Vesicula seminalis.
♂, Chitinous copulatory organ.
♂♀,  Common sexual aperture.
bc, Bursa copulatrix.

Reproduction.—All Turbellaria are hermaphrodite, and reproduce sexually, but a few forms (Microstomidae and some Triclads) increase during the summer by fission and during the winter by eggs. The body of the Microstomidae becomes constricted and partially subdivided into two, the posterior half regenerates a brain and pharynx. Subsequently each becomes again converted into two zooids, and the process is repeated until a chain is formed as in fig. 10. This breaks up into its constituent members, each of which repeats the process until the onset of reproduction. The Triclads, on the other hand, fragment, without undergoing preparatory changes.

The male and female genital ducts (gono-ducts) open to the exterior, either through a common chamber on the ventral surface (most Rhabdocoelida and all Tricladida, figs. 12, 14) or by separate apertures that are also usually ventral In the latter case, the male gonopore is usually in front of the female one (all Polycladida and some Rhabdocoelida). A separate opening is sometimes acquired by one or other of the accessory reproductive organs (as by the spermotheca in some Rhabdocoelida in which it is dorsal).

The generative organs of the Planarians are complex. Male and female germ-cells develop in one and the same individual and reach the exterior by independent ducts. These ducts are provided with accessory glands along their course and terminate in penial or vaginal structures, often of great complexity, which are surrounded by an “atrium” or invagination of the ventral body-wall. From this invagination a special vesicle “uterus” is often deeloped for the reception of the fertilized egg previous to oviposition.

EB1911 Planarians - Plan of an Alloeocoelous Turbellarian.jpg

Fig. 13.—Plan of an Alloeocoelous Turbellarian. Lettering as in fig. 12.

EB1911 Planarians - Plan of a Tricladid.jpg
Fig. 14.—Plan of a Tricladid.
i₁, Anterior, and i₂, i₃, paired posterior branches of intestine.
od Oviduct.
te, Tentacle.
vd, Vas deferens.
♂, Male, and ♀, female copulatory organ. Other letters as in fig. 12.

The Acoela present the simplest arrangement. In this group (fig. 11) the male germ-cells arise in follicles each of which is the product of a single sperm-mother-cell. From these follicles, the motile spermatozoa enter the paired sperm-duct, which opens by a single aperture near the hinder end of the animal, and is provided with a simple unarmed glandular penis. The female germ cells or ova arise from a paired ovary, some of the cells of which appear to act as nurse cells, supplying the young eggs with nourishment. When mature the eggs are transferred to the oviduct. At the point where the two oviducts join in order to open to the exterior they receive a conical sac (spermotheca) which contains spermatozoa. At this point the eggs are fertilized, and deposited in a mucilaginous mass which is attached to algae or buried in the sand. It is characteristic of the Acoela that the testes and ovaries should not be continuous with either the sperm-duct or the oviduct respectively.

In one genus of the Acoelous Turbellaria—Polychaerus—this primitive arrangement undergoes a development which foreshadows the complicated ovaria and vitellaria of higher forms. In Polychaerus the eggs mature in a special roomy chamber and are here provided with yolk which is elaborated by a sterile part of the ovary. Thus we have a differentiation of germ-cells into two portions allocated to two chambers: fertile ova which open eventually into the oviduct, and sterile ova that become yolk-cells and open into the brood-pouch.

The remaining Rhabdocoelida possess separate ovaries and yolk-glands. The union between the two sets of ducts takes place in the genital atrium which is provided with a spermotheca for the fertilization of the ova, but in at least one sub-family (Cylindrostominae) the spermotheca opens by a special dorsal pore. These ova, together with the yolk and spermatozoa, are then transferred to another atrial diverticulum—the uterus, in which a shell is formed and from which they are deposited in the form of a cocoon. In addition, a muscular pouch, the so-called “bursa copulatrix,” is usually present. The male organs of Rhabdocoelida are no less complex. The testes are either follicular (Alloeocoela) or compact (Rhabdocoela), and communicate indirectly or directly with the paired seminal ducts. The ducts unite at the base of an evaginable penis. This muscular organ is provided with glandular and chitinoid appendages of considerable complexity, and, in addition to these, a poison gland and duct are sometimes present. In certain genera (Macrorhynchus, Prorhynchus) the penis is used for catching prey. Perhaps exclusively so in the former genus. The opening of the atrium into the oral cavity in Cylindrostominae and of the male organ into the mouth of Prorhynchus is possibly explained by this fact.

EB1911 Planarians - Plan of a Polycladid.jpg
Fig. 15.—Plan of a Polycladid.
cn, Brain.
i, Intestinal branches.
i₁, Anterior unpaired intestinal branch.
ln, Longitudinal nerve cord.
m, Mouth.
od, Oviduct.
ov, Ovarian follicle.
ph, Pharynx.
ph₁,  Pharyngeal pouch.
st, Stomach.
t, Testicular follicle.
u, Uterus.
vd Vas deferens.
♂, Male copulatory organ, with the male aperture behind.
♀, Female copulatory organ, with the female aperture before it. The eyes are omitted.

From the Alloeocoela we pass readily to the Triclads. In both of these groups the reproductive organs are based on the same plan; but in Triclads the separation of ovarian and vitellarian portions of the gonad is less perfectly effected. The oviduct transmits the eggs from the anteriorly placed ovary, and receives in its course the openings of numerous vitellaria (fig. 14). No distinct spermotheca is developed, but a cocoon is formed in a special the uterus—which may either be a dilatation of the common oviduct (vagina) or of the atrium, and may open to the exterior independently (single in Uteriporus, paired in Syncoelidium). In Bdelloura the uterus is said to act as a spermotheca. In addition to these structures, accessory muscular organs are found in Dendrocoelum and developed to a high degree in land Planarians, where they form the so-called adenocheiri and adenodactyli (see von Graff, 1899).

Lastly, the Polyclads offer certain distinctive sexual characters. The ovaries are follicular, very numerous, and the ova elaborate their own yolk (fig. 15). The oviducts open into a chamber which, after receiving a voluminous shell-gland, opens by a muscular bursa to the exterior. No special uterus is developed, but from the point of union of the two egg-chambers a vesicle is given off which may open separately to the exterior (Trigonoporus). The testes are equally diffused and the seminal vesicles usually form a median muscular reversible sac which opens in front of the female genital pore. In Stylostomum, however, this penial organ opens through the mouth, as in certain Rhabdocoelida. Moreover, it may be paired (Thysanozoon) or multiple. Thus in Anonymus twelve or more pairs occur. In Cryptocelides two, four or six may be present, but in this genus they all lie in a common sac. In Polypostia twenty pores occur ranged about the female pore, but the most posteriorly placed of these structures are devoid of a seminal duct. This condition supports the view that in Polyclads the penis was at first a glandular organ probably used for attacking prey and that it has become secondarily connected with reproduction. In confirmation of this conclusion we have the observations of Lang (5) that Yungia stabs the body of other Polyclads with its penis when brought into contact with them. (See Whitman [9].) The genus Laidlavia differs from all other Polyclads in possessing a dorsal genital opening.

EB1911 Planarians - Larva of Yungia aurantica.jpg
(After A. Lang.)

Fig. 16.—Larva of Yungia aurantica, L. (Polycladida), with provisional ciliated processes.

Development.—The development of the Planarians is fairly well known. Except for one or two species of Polyclads, development is direct and without metamorphosis; but in Thysanozoon and Yungia the embryo develops eight strongly ciliated lobes which form a circumoral band of larval processes. These have been compared with the girdle of Trochosphere larvae and also with the eight rows of swimming plates in Ctenophores. From the name of their discoverer these girdled larvae are called Müller's larvae (fig. 16).

In the Rhabdocoelida the eggs are usually laid in a shell which has characteristic shapes. Each capsule contains a single ovum and several yolk-cells. Segmentation results in the formation of dislocated megacytes and microcytes. The latter give rise to the epidermis which is laid down in bilateral sheets, the former to the various internal organs. There is no distinction of germ-layers, and the gut is graduallly organized from the mesenchyme, the rest of which gives rise to the parenchyma. The pharynx and the rudiment of the gonads are the first organs to appear (Breslau [13], 1905). The development of the Acoela differs in certain particulars from that of other Rhabdocoelida. The ova contain yolk-granules, and yolk-cells are absent. Groups of such eggs, each with its own shell, are laid in a gelatinous envelope. Each ovum segments into a two-layered embryo composed of a ciliated outer layer and a central syncytium. No trace of a distinct enteron or gut is visible, but as the embryo grows the syncytium becomes differentiated into a more fluid central portion and a firmer peripteral zone. The former, together with the wandering phagocytes, corresponds functionally to the separate gut of other Rhabdocoelida. Pelagic larvae with a coat of long cilia have been identified by Uljanin as belonging to the Acoela.

The development of the Tricladida offers other peculiarities. From four to twenty or more ova are surrounded by several hundred amoeboid yolk-cells in each cocoon. Each egg-cell divides; but, as happens in the capsular ova of certain Mollusca and Oligochaeta, they do not all survive, some being used up as food by the remainder. The segmented ovum becomes dislocated as in some Rhabdocoels, the blastomeres moving apart from one another. The details of organ-formation are still imperfectly understood.

The eggs of the Polyclads are laid somewhat like those of the Acoela in a gelatinous enveloule, each ovum being provided with yolk and an egg-shell which may be operculate. The majority of species go through a direct development. The segmentation of the egg in Discocelis and Leptoplana has been worked out by Lang and his results re-interpreted by Wilson and others (Hubrecht [12]). In Polyclads a distinction of germ-layers similar to that occurring in the development of Mollusca, Chaetopod-Annelids and certain other Invertebrates, is early apparent. The ovum by unequal segmentation gives rise to megameres and micromeres, and between the two, intermediate cells form one origin for the mesenchyma. The micromeres surround the intermediate and centrally placed macromeres. The latter undergo division into hypoblast cells and yolk-masses. The similarity of cell-lineage in Polyclads and Coelomate Invertebrates, together with the trochosphere-like Polyclad larval form (Müller's larva), have been the two chief arguments in support of the view that this group is a link between the Planarian and Coelomata. It is at present, however, doubtful whether such highly organized animals as Polyclads can be regarded as in any sense ancestral forms. Their relations to other Turbellaria are quite uncertain, and on present evidence it seems legitimate to hold that they are the most highly differentiated division both in embryonic and adult structure.

Systematic Arrangement.

Order Turbellaria.—Free-living Platyelmia with a ciliated epidermis. A well-developed nervous system and sense-organs concentrated at the anterior end of the body, diffused elsewhere.

Sub-order A. Rhabdocoelida.—Gut syncytial or tubular. Female gonads always compact.

Tribe I. Acoela (fig. 11).—Mesenchyma not differentiated into separate gut and parenchyma. No excretory organs of protonephridial type. A simple pharynx. A median otocyst (statocyst) over the brain. Small, often flattened forms. All marine and many infected by brown or green algal cells. One species parasitic in Echinoderms.

Tribe II. Rhabdocoela (fig. 12).—Gut and parenchyma separate, the former a simple straight sac. Vitellarla usually present. Testes compact. Penis and pharynx often complex, occasionally protruded through a common opening. Marine and fresh-water. Many fresh-water forms infected by algal cells. Typhloplana, Graffilla, Anoplodium, are respectively parasitic in Nephthys, in Gastropods and Holothurians.

Tribe III. Alloeocoela.—Gut and parenchyma distinct. Intestine straight or lobate. Testes follicular. Penis and pharynx simple. One family with otolith. All marine except Plagiostoma lemans (deep-water, Geneva) and the Bothrioplanidae.

Sub-order B. Dendrocoelida.—Large forms with flattened body, branched intestine, follicular testes and follicular ovaries or compact ovaries and yolk-glands.

Tribe I. Tricladida.—Intestine with three main branches. A pair of compact ovaria and numerous yolk-glands connected by a common duct. A single genital aperture. Fresh-water forms: Planaria, Dendrocoelum, Polycelis, common. Peculiar forms in Lake Baikal. Marine forms: Gunda segmentata, Bdelloura (external parasite of Limulus). Terrestrial forms: Rhynchodemus, Geoplana, Bipalium.

Tribe II. Polycladida.—Body leaf-like. Intestine composed of a median stomach with many branched or reticulated coeca; testes and ovaries follicular; genital openings usually separate, the male gonopore preceding the female one. Multiple male gonopores in some forms. All marine and widely distributed; some genera cosmopolitan.

Literature.—(1) L. von Graff (Rhabdocoela, Acoela, Tricladida), Monographie d. Turbellarien (1882), vol. i., (1899) vol. ii.; Die Acoela (1891); (2) Arbeiten aus der zool. Institut zu Graz (1904, 1905, 1906); (3) “Turbellaria,” in Bronn's Klassen v. Ordnungen d. Thierreichs, vol. ii.; (4) Turbellaria als Parasiten v. Wirthe (Graz, 1904); (5) A. Lang, “Die Polycladen,” Fauna and Flora of the Gulf of Naples, vol. ii. (1884); (6) F. F. Laidlaw (Polyclads) in Zoological Results of Expeditions conducted by Dr Willey, Stanley Gardiner and C. Crossland, Cambridge Univ. Press, and Proc. Zool. Soc. (1902-1906); (7) Gamble and Keeble (Green cells of Convoluta), Quart. Jour. Micro. Sci. (1903, 1907); (8) E. R. Pearl (Bionomics of Planarians), ibid. (1903); (9) Whitman (Hypodermic Impregnation), Jour. Morphology (1890), iv. 361; (10) Hesse (Eyes of Planarians), Zeitschr. f. wiss. Zool., vol. lxi. (1897); (11) Carrière (ditto), “Die Sehorgane der Thiere” (1885); (12) A. A. W. Hubrecht (affinities), Zeitschr. f. Naturwiss. (Jena, 1905); (13) Breslau (Development of Rhabdocoels), Zeitschr. f. wiss. Zool. (1905). Besides these special works, useful general accounts of the Turbellaria will be found in Cambridge Natural History, ii. 1-50; A Treatise on Zoology (Black), iv. 1-42, and the references given by these works.

Appendix to the Turbellaria.

Class Temnocephaloidea.—This small class of Platyelmia possesses a special interest. It connects the Turbellaria (and in particular the Vorticid rhabdocoela) with the Trematoda. At the same time the Temnocephaloidea present certain peculiar structural features which entitle the class to an independent position.

The name of the class is derived from the digitate tentacles which occur on the anterior or lateral margins of the body. The body measures about 5 mm. in length, and the flattened ventral surface is armed with a sucker. It presents in most genera the appearance of a minute cephalopod, but in Craspedella the posterior part of the dorsal surface is raised up into three transverse fringed lamellae. These animals are epizoic, i.e. they live attached to the outer surface of other organisms, but are not ectoparasitic for they ingest Infusoria, Rotifera and Diatoms. Most of the species occur on fresh-water crayfish and crabs in Chile, Madagascar, the Malay Archipelago and Australasia. Two Brazilian forms are known, one from the pulmonary chamber of the Mollusc Ampullaria and the other from water tortoises. The genus Temnocephala is found in all the countries mentioned. The two others, Craspedella and Actinodactylella are only known from Australia.

EB1911 Planarians - anatomy of Temnocephala.jpg

(From Cambridge Natural History, vol. ii. “Worms,” &c., by permission of Macmillan & Co., Ltd.)

Fig. 17.—The anatomy of Temnocephala (after Haswell).

♀♂,  Common genital aperture. rh Rhabdites.
d Gut. rh.c Rhabdite-forming cells.
ex.s Excretory sac. sc Sucker.
m Mouth. sh Shell-gland.
ov Ovary. te Testis.
ovd Oviduct. ut Uterus.
ph Pharynx. vg Vagina.
rv Receptaculum vitelli. vs Vesicula seminalis.

The epidermis offers an interesting transitional structure. It is still, as in Turbellaria, cellular, or rather syncytial without cell-boundaries, but in most species has lost its cilia and developed a thick cuticle as in Trematodes, pierced by the necks of subdermal gland-cells. These cells however, still retain the Turbellarian character of secreting rhabdites and form thickly-grouped tracts. The mouth, which is placed near the anterior end, leads into a bulbous pharynx from which a short, broad digestive sac is given off. The excretory system is peculiar. Besides the ordinary flame cells, single large canaliculated cells may form the commencement of the tubules, composed of comparatively few cells with large nuclei. They open to the exterior by a pair of contractile sacs situated dorsally at the level of the mouth as in certain Trematoda. Each sac is the product of a single cell, and is said to contain several branches of “flames” or synchronously contractile cilia. The reproductive system recalls that of certain Rhabdocoels, whilst the nervous system has retained a more primitive condition. The brain, which is placed over the mouth, gives rise to six main longitudinal tracts interconnected by a subdermal network. A pair of eyes is placed above the brain

Class and order Temnocepholoidea.—Platyelmia in which the flattened body is produced into anterior or anterior and lateral tentacular processes and carries a ventral sucker. The epidermis is a syncytium covered by a thick cuticle. Cilia and rhabdites are present. Family I.: Temnocephalidae: 4-12 anterior tentacles. Family II.: Actinodactylellidae. Lateral tentacular processes.

See Haswell, Macleay Memorial Volume (1893); Plate, Sitzberich. Akad. Wiss. Berlin (1894), p. 527. (F. W. Ga.)

  1. These references are to the literature at the end of this article.