1902 Encyclopedia > Planarians


PLANARIANS. The name Planaria was first applied by O. F. Müller in his Frodromus Zoologix Daniese (1776) to a group of worms, inhabitants of fresh and salt water, characterized, so far as was then known, by a flattened leaf-like form. Ehrenbergin 1831 changed this name to Turbellaria on account of the cilia with which the body is furnished, by means of which the worms create a whirlpool in the surrounding water. The extent of this group was subsequently more restricted, and at present the name Turbellaria is applied to all those (mainly free-swimming) Platyhelminths whose body is clothed externally with a ciliated epidermis (fig. 9), and which possess a mouth and (with the exception of one division) an alimentary canal, but are without an anus. The Turbellarians, exclud-ing the NEMERTINES (q.v.), which until recently were classed with them, form an order of the class Platyhelminthes, and the old name Planaria is now confined to a group of the fresh-water representatives of this order.

Size and External Characters. — Many forms of the Turbellarians are so minute as to be hardly visible with the naked eye, while others attain to a length of several inches, and a land Planarian of no less than 9 inches in length has been described by Moseley. The freshwater forms are generally small, the largest representatives of the order being marine or terrestrial. The smaller species are mostly cylindrical, or convex dorsally and flat ventrally; the anterior extremity is commonly trun-cated and the posterior extremity pointed (fig. 1, a, b). The larger aquatic forms are thinner in proportion to the increasing surface of the body, so that they come to resemble thin leaf-like lamellae (d), while the large land Planarians instead of increasing in superficies grow in length (e and /), so that they may be best compared to leeches. The larger aquatic forms are frequently provided with tentacles in the shape of paired finger-like processes or ear-like folds of the anterior part of the body (d and g); sometimes the tentacles are papillary outgrowths of the dorsal surface; the land Planarians are often I " !? 6 { to be distinguished by a | crescent-shaped area at the fore end of the body, which is separated off from the rest (/). In many cases the whole dor sal surface is beset with papillae (d). The aper-ture of the mouth varies greatly in its position; sometimes it is situated at the anterior extremity, sometimes in the middle of the ventral surface of the body, occasionally quite close to the posterior ex-tremity; the single com-mon or distinct male and female generative aper-tures are also situated upon the ventral surface of the body, and the former in rare cases open in com-mon with the mouth ; the genital apertures always lie behind the mouth. Many Turbellarians have a sucker which serves to attach the animal to sur-rounding objects, or to another individual during copulation..

Integument.—The integument is composed of a single layer of ciliated epithelium; between the cilia there are-often long flagella and stiff tactile hairs and even (in a single instance) chitinous spines; these latter must be-regarded as local thickenings of the firm cuticle which covers the epidermic cells. The epidermic cells are flat or columnar, and are united to each other by smooth opposed margins or by denticulate processes which fit into similar processes in the adjacent cells (fig. 2). Sometimes the epidermic cells are separated by an interstitial nucleated tissue. The structure and functions of the cells of the epidermis differ, and four varieties are to be found :—(a) indifferent ciliated cells; (6) cells containing certain definite structures (rhabdites, nematocysts); (c) gland cells; and (d) glutinous cells (Klebzellen). The rhabdites are refracting homogeneous rod-like bodies, of a firm consistency, which are met with in most Turbellaria, and often fill all the cells of the epidermis ; they are not always found entirely within the cells, but the extremity often projects freely on to the exterior of the body. They are readily extruded from the cells by pressure, and are often found in great abundance in the mucus secreted by the glandular cells (many Turbellarians, like snails, deposit threads of mucus along their track) ; in this case the epidermic cells become perforated like a sieve. In many Turbellarians the rhabdites are chiefly massed in the anterior part of the body; frequently there are several varieties of rhabdites in one and the same species,—some being pointed at both ends, others cylindrical with truncated extremities. These structures are either formed directly in the ordinary epidermis cells as a kind of secreted product of the cell, or in special formative cells which lie beneath the integument and are-connected with the epidermis cells by protoplasmic fila-ments, by means of which the rhabdites reach the surface of the body. These cells must be regarded as epidermic- cells which have become disconnected with the epidermis itself, and wandered into the subjacent parenchyma. The function of the rhabdites seems to be to support the tactile sense. In rare instances nematocysts are present which in structure and development entirely resemble those of the Ccelentera (see vol. xii. p. 550). Very com-monly structures known as pseudo-rhabdites are present; these have a rod-like form, but instead of being homo-geneous are finely granular; they are an intermediate step between the rhabdites proper and a granulated secretion occasionally thrown off by the gland cells. The unicellu-lar glands are either situated among the epidermic cells or in the parenchyma, in which case they are connected with the exterior only by the excretory duct. A peculiar modi-fication of the epidermic cells are the so-called "glutinous cells," which occur on the ventral surface or at the hinder end of the body of many Turbellarians, and compensate for the suckers; the surface of these cells is furnished with numerous minute processes by means of which and a sticky secretion the animals can attach themselves to sur-rounding objects. Sometimes the epidermic cells contain calcareous concretions, and very commonly pigment is found either in the cells themselves or within the inter-stitial tissue. The colours of Turbellarians are, however, not always due to the pigment of the epidermis but to pigment contained in the parenchyma. Beneath the epidermi* is a basement membrane (fig. 2, bm) which is in

FIG. 2.—Integument of Mesostoma lingua, O. Sch. On the right hand is the epidermis (z) with perforations (/) through which the rhabdites (st) project. Beneath this the basement membrane (6m), and beneath this again the muscular layers consisting of circular (cm), diagonal (sm), and longitudinal (lm) fibres.
some cases very delicate and structureless, and in other cases much thicker and enclosing branched cells; this membrane is attached more firmly to the subjacent tissue than to the epidermis. Since this tissue is the strongest in the body, and serves as a surface of attachment for the muscles, it has been termed by Lang a skeletal membrane.
The third section of the integument is formed by the muscular layers. These form a continuous covering to the rest of the body, but their arrangement and thickness are very different in different forms. In the smaller species (Bhabdocoslida) there are two layers, an outer circular and an inner longitudinal, only in a few cases the circular layer is external to the longitudinal; sometimes there are three distinct layers, as in fig 2, where a diagonal layer is inter-posed. The larger forms (Dendroccelida) have a much more complicated muscular system : in the most differen-tiated forms there are six separate layers (two circular, two diagonal, and two longitudinal), which are, however, always less developed upon the dorsal than upon the ventral surface in that the thickest layer of the ventral surface (the innermost longitudinal) is absent or very feebly developed upon the dorsal side. Besides the integumentary muscular system, there are also found dorso-ventral muscular bands which traverse the whole body from the dorsal to the ventral basement membrane, being branched at both extremities, and the special muscles of the pharynx, genital organs, and suckers.

The perivisceral cavity, bounded by the integument and traversed by the dorso-ventral muscles, contains the organs of the body—alimentary canal, excretory system, nervous system, and genital glands. The space left between these organs is filled with parenchyma ; the latter varies much in appearance and is very difficult to study. Generally it consists of a network of fibres and trabecular which contain nuclei, and between which is a system of cavities filled during life with the perivisceral fluid. These cavities are generally but few in number and vary with the stronger or feebler development of the reticulum -r they occasionally contain free cells.

Alimentary Canal.-—All Turbellarians are furnished with a mouth, which, as there is no anus, serves both to take in nutriment and expel the undigested remains of food. The alimentary canal consists of a muscular pharynx and an intestine. The pharynx (figs 3, 5 to 8, ph} is cylindrical in form, rather complicated in structure, and surrounded by a muscular sheath, which opens on to the exterior by the mouth (m). Often the pharynx consists, merely of a circular fold lying within the pharyngeal pouch (fig. 8); it can be protruded through the mouth and acts like a sucker, so that the animal can fasten itself upon its prey and draw it into the intestine by suction. At the junction of the pharynx with the intestine open the salivary glands, which are frequently large and well-developed (fig. 5, s). The intestine (i) has a very characteristic form in the different sections, and has long served to divide the Turbellaria into two groups:—(1) Bhabdoccelida, with a straight unbranched intestine (figs. 5r 6), and (2) Dendroccelida, with a branched intestine (figs. 7r 8). In the latter group Lang has recently called attention to further differences that exist in the form of the intestine : in the Tricladida (fig. 7) there is no central "stomach," but three equally-sized intestinal branches (which have secondary ramifications) unite together to open into the pharynx; in the second group, the Polycladida (fig. 8), there is a median stomach (si), from which numerous intestinal branches arise; this stomach communicates directly with the pharynx; the branches of the intestine are much ramified and often form an anastomosing net-work. The epithelium of the intestine is a single layer of cells generally not ciliated, capable of protruding amoeboid processes by which the food is absorbed; the digestion of these animals is intracellular. Sometimes a muscular coat surrounds the intestine, the lumen of which is thus capable of being totally or partially contracted. To the-above-mentioned divisions of the group, distinguished from each other by the varying form of the alimentary tract, another has been added, viz., the Accela (Ulianin),. which are characterized by the entire absence of any intestine. In these forms (fig. 4) the mouth leads directly into the parenchyma of the body by a short tube which is. merely an invagination of the integument; the paren-chyma is a syncytium, consisting of a soft protoplasmic mass with scattered nuclei, which represents the elements of the intestine and the body parenchyma (ento- and mesoderm) completely fused and without any traces of differentiation. This fact, as well as the disappearance of a nervous and excretory system, reduces the Accela to the lowest position not only among the Turbellaria, but among the whole group of the Vermes.

Excretory System.-—The excretory system of the Turbel-larians is quite similar to that of the Trematodes and Cestoids; it consists of (1) the main trunks with their _external aperture, (2) the secondary branches of these, and (3) the excretory cells with the fine tubules leading from them. Rarely is there but a single main excretory trunk present opening at the hinder end of the body (Steno-stoma); generally there are a pair of such trunks which open in common at the hinder end of the body, or separately (most Rhabdocmla), or by the mouth (fig. 3). In the Tricladida there are two or four lateral trunks present which open by a number of pores arranged in pairs upon the dorsal surface of the body; the same appears to be the case in the Polycladida. The main trunks of the excretory sys-tem are generally much twisted in their course, and anastomose with each other; they receive the fine tubules either directly or, as in the Rliabdocada, there is a network of secondary tubules interposed. The excretory cells are pear-shaped; they are branched and furnished with a nucleus and a large vacuole which is directly continuous with the lumen of the tubule; from the boundary wall of the vacuole springs a single flagellum, which depends into the lumen of the tubule and is capable of active movement. Lang discovered in a marine form of the Tricladida (Gunda) similar vacuo- re-lated Cells with a single flagellum

among the epithelial cells of the » — ~'

intestine, and came to the conclu-sion that the excretory cells were on that account derived from the epithelium of the intestine. The movements of the excretory fluid towards the external pore are directed by this flagellum as well as by cilia developed upon the walls of the fine tubules; the motion of all these cilia is such as to drive the contents of the tubules towards the excretory pore. The main trunks of the excretory system are either sparsely (Tricladida according to Jijima) or com-pletely (Polycladida according to Lang) lined with cilia.

Nervous System.—The central organ of the nervous system, the brain (cn), is a double ganglion at the anterior end of the body, and has been noticed in all the known forms with the exception of the Acoela. It is situated in front of or above the pharynx; in those species in which a process of the intestine extends beyond the region of the brain (cf. figs. 7 and 8 viewed from the ventral surface) it is placed below this. In such cases there is sometimes a com-missure encircling the prolongations of the intestine. Each of the two ganglia gives off a strong longitudinal nerve cord (figs. 5-8, In) from which arise branches going to the various organs of the body. The structure of the nervous system is somewhat different in the Phabdoccela, Tricladida, and Polycladida. In the first group (figs. 5, 6) the two longitudinal cords and their branches are the most feebly developed, and there is but rarely (Mesostoma, Monotus) a transverse commissure uniting the longitudinal cords. These cords are very large in the Tricladida, where the brain is to be regarded as a simple thickening of them; in this group there are numerous transverse commissures between the longitudinal nerve cords (fig. 7), and the nerves arising from them and passing to the periphery form a subcutaneous nerve plexus within the muscular coat. Lang has observed a similar nerve plexus in the Polycladida, the central nervous system of which differs from that of the Tricladida in that a number of stout nerve cords radiate outwards from the brain as well as the

two longitudinal cords; they are all united together by

Fig. 4.

FIG. 4.—Plan of an Accelous Turbellarian. e, eye; m, mouth; ot, otolith; ov, ovary; p, digesting parenchyma; t, testicular follicles ; vs, vesicula .*eminalis J , male organ of copulation ; $ 9 , common sexual aperture.

FIG. 5.—Plan of a Rhabdocoelous Turbellarian. be, bursa copulatrix ; ai, brain; e, eye; g, germarium; i, intestine; In, longitudinal nerve trunk; rn, 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; S 9 , common sexual aperture; 6c, bursa copulatrix.
numerous commissures, and a network is thus formed which extends throughout the body.

auditory organs (otoliths), and eyes. The whole surface of the body is very sensitive and (e.g., in the Polycladida) con-tains cells which end in tufts of fine hairs, so that certain regions thus become specially sensitive and serve as tactile organs. The anterior pointed extremity of the body in the Rhabdocmla is characterized by an abundant development of rhabdites and tactile hairs, and thus becomes a special tactile organ; in other cases this region of the body is transformed into a conical tactile proboscis which can be retracted into a sheath (Proboscida). In the freshwater Tricladida the anterior margin of the head is richly inner-vated, and is beset with a special row of tactile cells which contain no rhabdites; in the terrestrial forms of the same family (Bipalium) Moseley has described a row of papillae along the crescent-shaped anterior extremity which can be

FIG. 8.—Plan of a Polyeladid. en, brain; i, intestinal branches; ii, anterior unpaired intestinal branch; In, longitudinal nerve cord; m, mouth; od, oviduct; ov, ovarian follicle ; pit, pharynx ; phx, pharyngeal pouch ; st, stomach; t. testicular follicle; u, uterus; vd, vas deferens; male copulatory organ, with the male aperture behind; Q, female copulatory organ, with the female aperture before it. The eyes are omitted.

extended and form tactile organs ; between the papillae are peculiar ciliated grooves connected with nerves. In the Polycladida there are tactile cells with stiff hair-like pro-cesses on the summit of the dorsal papilla? and the various tentacular structures; the tentacles in this family also serve to support the eyes.

The majority of the Turbellarians possess eyes; the Rhabdoccelida commonly have two or four, as also have the Tricladida; the latter, however, are in some instances furnished with a greater number arranged in a continuous row round the anterior end of the body; in the Poly-cladida there are from fourteen to several hundred eyes arranged in two symmetrical groups round the brain or scattered over the whole of the anterior margin of the body and upon the tentacles. The eyes are always situ ated beneath the integument within the parenchyma, sometimes directly upon the brain or connected with it by special optic nerves. In its simplest form the eye is a pigmented spot with or without a refractory lens-like body ; the more complicated eyes consist of a pigmented sheath containing a number of refracting rods which are connected at their outer extremity with a series of retinal cells, one to each rod; the retinal cells are prolonged into a nerve thread running to the brain; the arrangement of the visual elements is therefore precisely the same as in the vertebrate eye. Of great interest is the fact that in the Polycladida the number of eyes increases with the growth of the animal, and Lang has shown that the eyes increase in number by actual division. On the other hand Carriere has discovered by experimenting with certain freshwater Tricladida that the compound eyes (those containing a number of rods) are formed by the coalescence of several simple eyes. Only a single eye is found in the Monotida, which has the form of a simple pigment spot in front of the otolith.

Auditory organs are found in the shape of vesicles filled with fluid and containing circular lenticular or spindle-shaped otoliths formed of carbonate of lime. Otolithic vesicles of this kind are found in many Rhabdoccelida (Acceia, Monotida, fig. 4, ot) embedded in a depression on the anterior surface of the brain. In the Dendroccelida these organs are but rarely present.

As a sensory organ of unknown function must be men-tioned the paired lateral ciliated grooves which are met with on either side of the brain in many Rhabdoccela (fig. 9, c); they are also found commonly in NEMERTINES (q. v.), but are here more complicated in structure.

Reproductive Organs.—With a few exceptions all the Turbellarians are hermaphrodite, and reproduce themselves sexually. Only among the Microstomida is there an asexual as well as a sexual reproduction. The male and female organs open to the exterior, either through a common cloaca (atrium genitale) on the ventral sur-face (most Rhabdoccelida and all Tricladida, figs. 4-7), or there are separate male and female apertures. In this case the male aperture is generally placed in front of the female aperture (some Rhabdo-ccelida and all Polycladida, fig. 8), but occasionally the positions are reversed (certain Rhabdoccelida). The genital glands display a primitive condition on being paired, though frequently the ger-marium (fig. 5, g) of the Rhabdoccela, and occasionally also the testis, is developed only upon one side of the body.

The structure of the female organs varies. In some cases there are simple ovaries (ov in figs. 4, 8) in which the ova originate and become fully mature without being furnished with the secretion of a second gland ; in other cases there is a division into ger-marium (fig. 5-7, g) and yelk gland (v); the primordial ova or germs originate in the former, and absorb the products of the yelk gland in the atrium, where they become ready for fertilization. An intermediate condition is seen in those forms where there is but a simple gland present which produces germs in its upper portion and yelk in the lower portion. The ovaries are generally compact round or tubular glands (fig. 4) ; sometimes they are formed of a number of pear-shaped follicles (fig. 8); there is usually a simple or paired uterus (u) which retains the ova for some time before they are deposited ; sometimes, however, the ova undergo their development within the uterus and are completely developed before expulsion ; in some cases the egg-shell is detached within the uterus so that the young are produced alive.

In Turbellarians without a yelk gland the uterus is a simple widening of the oviduct (fig. 8); in those forms which possess addi-tional yelk glands the uterus is a simple or paired diverticulum of" the atrium genitale (figs. 5, 7). The ova are either surrounded by a more or less hard chitinous shell, or one shell contains a number of ova ("cocoon" of Tricladida and many Polycladida). The Polycladida deposit an egg-string which like that of the Gastropoda consists of a number of eggs bound together by a transparent albumen-like mass. Many Ehabdoccel Turbellarians (e.g.,Mesostcma, ehrenbergii) produce two sorts of ova, thin-shelled summer ova and thick-shelled winter ova ; the latter are capable of withstanding a considerable amount of desiccation, and are deposited in the autumn. The accessory female organs of reproduction are represented by bursa; seminales, which receiver the semen during copulation and retain it until fertilization is accomplished. A further division of labour is brought about by the presence of two diverticula of the atrium genitale, one of which serves as a bursa copulatrix (fig. 5, be) and the other as a receptaculum seminis (rs) in the same sense as the equivalent organs of insects. In the place of a special receptaculum seminis the efferent duct of the ovary is often (Mesoslomida) metamorphosed into a chamber to contain the semen. In the Tricladida and Polycladida the female efferent duct is often differentiated into a muscular vagina which closely resembles the penis (figs. 7, 8, 2 ).

Finally, the female generative apparatus is furnished with a number of glands which have been termed cement glands, albumini-parous glands, and shell glands.

The male sexual glands (figs. 4-8, t) resemble the ovaries in being either compact tubular (fig. 5) or follicular (figs. 4, 6, 7, 8) struc-tures. The vasa deferentia (vd) are often widened out into vesiculae seminales (figs. 4, 6, TO) ; or there are special vesiculae seminales present, formed by a portion of the penis (fig. 5, TO). In the male organ of copulation there is frequently found in addition to the spermatozoa an accessory granulated secretion produced by special glands, but of unknown function.

The muscular penis, especially in the Rliabdoccela, has a number of chitinous spines and hooks which serve to assist the animal in maintaining a firm hold during copulation, but also in capturing and retaining its prey. In Macrorhynchus helgolandicus, Gff., there is a peculiar poison dart connected with the male copulatory organ which only serves the latter purpose. Very remarkable is the opening of the penis into the mouth cavity in Stylostomum (Polycladida) and Prorliynchas (Rhabdocmla), and also the existence of several (2-15) pairs of male copulatory organs and genital apertures in certain Polycladida.

The spermatozoa vary much in form, especially in the Rhabdo-ccelida, where frequently the species of one and the same genus are distinguished by the different form of the spermatozoa. Copulation in the Turbellarians is generally reciprocal; only in those cases where both summer and winter ova (see above) are formed do the former arise from self-fertilization; the latter are the result of the copulation of two individuals. The fertilization of the ova always takes place in the atrium genitale. Many Turbellarians, especially the Accela, display the phenomenon known as " successive her-maphroditism, the male organs of an individual attain to maturity first, and the female organs become ripe subsequently. During copulation, therefore, one individual is physiologically a male and the other a female.

Asexual generation is met wdth only in the Microstomida; it takes the form of transverse division accompanied by budding. The posterior third of the body becomes separated off by a septum running from the gut to the integument and an external furrow corresponding to this; this part of the body grows in length until it equals the anterior portion. By further repetition of this double procedure of separation and equalization there, chains of 4, then 8, 16, and 32 buds are formed, which remain attached (fig. 9), and, although fresh mouth apertures (m', m", in'") have been formed, are still in communication by the intestinal lumen; this becomes closed before or after the several buds break off from their connexion with each other. Throughout the whole summer chains of zooids are met with; in autumn this asexual division probably ceases to occur; the several individuals become sexually mature, separate from each other, and lay eggs which remain quiescent during the winter and in the spring develop into fresh individuals reproducing asexually.

Development. —The study of the development of the Turbellarians is unfortunately not very far advanced, particularly among the small Rhabdoccelida, which are extremely difficult to investigate, and about which hardly any developmental facts are known. The larger freshwater Tricladida and the Polycladida on the contrary have been recently very fully investigated. The Rhabdoccela and the Tricladida appear to develop directly without any metamor-phosis, while a great part of the Polycladida undergo a metamor-phosis and pass through a larval condition, during which they are furnished with provisional ciliated processes (fig. 10); the Accela have also a free larval form; pelagic larvae with a coat of long cilia apparently belonging to this group have been observed by Ulianin. The segmentation of the ovum is total, but unequal; an epibolic gastrula is formed and the aperture of invagination becomes the permanent mouth of the adult.

Systematic Arrangement and Mode of Life.—Order Turbellaria.— Platyhelminths with a ciliated integument, a mouth and pharynx, but no anus ; with paired cerebral ganglia and two lateral nerve cords ; sexual organs hermaphrodite; chiefly free-swimming.

Sub-order A. Rhabdoccelida.—Of small size; body cylindrical or depressed ; without an intestine, or with a simple unbranched intestine; the female genital glands always compact, not follicular; genital apertures single or distinct.

Tribe I. Accela (fig. 1, a,).—With a digestive parenchyma not differentiated into intestine and parenchyma proper; with no nervous system or excretory organs ; sexual organs hermaphrodite, with follicular testes and paired ovaries : generally without a pharynx, but having otoliths ; all the forms marine. Many quite flat, with the lateral margins bent down towards the ventral surface (Convoluta), frequently with brown or green parasitic algae in the parenchyma.

Tribe II. Rliabdocoela (fig. 1, b).—Intestinal tract and paren-chyma separate ; nervous system and excretory organs present ;

Fig. 9.
with compact testes and female generative glands (ovaries or separated germarium and yelk glands) ; with a complicated pharynx, but generally without otoliths. Numerous forms, freshwater and marine ; the genus Prorhynchus (two species) also in damp earth. The Microstomida (fig. 9) propagate asexually.

Fig. 10.

FIG. 9.—Microstoma Hneare, Oe., undergoing division. There are 16 individuals, 8 with mouth apertures, showing the buds of the first (m), second (fn7), third (m"), and fourth (m'"> generation. The fifth generation has not yet acquired a mouth aperture, c, ciliated grooves; e, eye spots ; i, intestine.

FIG. 10.—Larva of Yungia aurantica, L. (Polycladida), with provisional ciliated processes (after A. Lang).

Freshwater forms mostly belong to the families Mcsostomida and Vorticida, some of which contain green parasitic algae. Marine forms include representatives of these two families and of the Proboscida (wdth a tactile proboscis). Of the family Vorticida, the genera Graffilla and Anoplodium are parasitic, the former in Gastropods the latter in Echinoderms (Holothurians).

Tribe III. Alloioceela (fig. 1, c).—Intestinal tract and parenchyma separate ; nervous system and excretory organs present; with folli-cular testes and compact female glands (as in the Rhabdoccela); pharynx similarly developed as a shorter or longer sac. One family (Monotida), with otoliths. All the species marine, with one excep-tion, Plagiostoma lemani, which lives in the deep water of the Alpine lakes.

Sub-order B. Dendroccelida.—Large forms, with a flattened body, branched intestine, follicular testes and follicular yelk glands or ovaries ; without otoliths.

Tribe I. Tricladida. — Body elongate; intestine with three main branches uniting to open into a cylindrical retractile pharynx ; with follicular testes, two round germariums, and numerous yelk follicles, wdth a single sexual aperture. Planaria, Dendroccelum, Polycelis (fig. 1, g) are inhabitants of fresh water (with great power of reproduction). Terrestrial forms (fig. 1, e, f) of leech-like shape, especially met with in the tropics (only two European species Rhyn-chodemus terrestris and Geodesmus bilineatus); marine forms Gunda (characterized by a metameric structure), Bdelloura (external para-site of Limulus).

Tribe II. Polycladida (fig. l,d).—Body leaf-like, thin, and broad, with numerous branched or retiform intestinal cceca which unite to form a central tube (stomach); with follicular testes and follicular ovaries, with two separated genital apertures, the male in front of the female ; without (Acotylea) or with (Cotylea) a sucker situated behind the female generative opening. All marine.

Literature.—The most recent works, which also contain a full account of what has gone before, are the following :—

Rhabdoccela.—L. v. Graff, Monographie der Turbellarien: 1. Rhabdoccelida, Leipsic, 1882, with 20 plates.

Marine and Freshwater Tricladida.—A. Lang, " Der Bau von Gunda segtnentata und die Verwandtschaft der Platyhelminthen mit Coelenteraten und Hirudineen," in Mitth. Zool. Stat. Seapel, vol. ill., 1881; El. Metschnikoff, "Die Embryologie von Planaria polychroa," in Zeitschr. f. wiss. Zool., vol. xxxviii., 188-3; Isao Jijima, " TJntersuchungen iiber den Bau und die Entwickelungsgeschichte der SOss-
"wasser-Dendrocoelen," in Zeitschr. f, wiss,Zool., vol. xl, 1884.

Land Planarians.—"H. N. Moseley, " On the Anatomy and Histology of the Land Planarians of Ceylon, with some Account of their Habits, and with a Description of Two New Species, and with Notes on the Anatomy of some European Aquatic Species," in Phil. Trans. (London, 1874), and " Notes on the Structure of several Forms of Land Planarians, with a Description of Two New Genera and Several New Species, and a List of all SpecieB at present known," in Quart. Jour. Micr. Sci., vol. xlvii., 1877 ; J. v. KenneL "Die in Deutschland gefundenen Landplanarien Rhynchode- mus terrestris und Geodesmus bilineatus* in Arbeit. Zool.-Zootom. Instit. Würzburg, v., 1879.

Polycladida.—A. Lang, " Die Polycladen," in Fauna und Flora des Golfes von Neapel, No. 1], 39 plates, Leipsic, 1884-85. (L. v. G.)

The above article was written by: Prof. L. von Graff, Ph.D.

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