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 truncated 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 <∕); sometimes the tentacles are papillary outgrowths of the dorsal surface; the land Planarians are often 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 (<Z). The aperture 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 extremity; the single common or distinct male and female generative apertures are also situated upon the ventral surface of the body, and the former in rare cases open in common with the mouth; the genital apertures always lie behind the mouth.

Many Turbellarians have a sucker which serves to attach the animal to surrounding 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 (J) 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 filaments, 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 Coelentera (see vol. xii. p. 550). Very commonly structures known as pseudo-rhabdites are present; these have a rod-like form, but instead of being homogeneous are finely granular; they are an intermediate step between the rhabdites proper and a granulated secretion occasionally thrown off by the gland cells. The unicellular 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 modification 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 surrounding objects. Sometimes the epidermic cells contain calcareous concretions, and very commonly pigment is found either in the cells themselves or within the interstitial 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 epidermis is a basement membrane (fig. 2, bm) which is in some cases very delicate and structureless, and in other eases 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 (Rhabdocoelida) 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 interposed. The larger forms (Dendrocoelidd) have a much more complicated muscular system: in the most differentiated 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 trabeculae, 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; 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) Rhabdocoelida, with a straight unbranched intestine (figs. 5, 6), and (2) Dendrocoelida, with a branched intestine (figs. 7, 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 (st), 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 network. 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 Acoela (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 parenchyma 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 Acoela to the lowest position not only among the Turbellaria, but among the whole group of the Vermes. Excretory System.— The excretory system of the Turbellarians 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 Rhabdocoela), or by the mouth (fig. 3). In the Trieladida 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 system are generally much twisted in their course, and anastomose with each other; they receive the fine tubules either directly or, as in the Rhabdocoela, 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 Trieladida (Gunda) similar vacuolated 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 (Trieladida according to Jijima) or completely (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 commissure 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 Rhabdocoela, 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 (Mesostoana, Monotus) a transverse commissure uniting the longitudinal cords. These cords are very large in the Trieladida, 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 Trieladida 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 numerous commissures, and a network is thus formed which extends throughout the body.

Sense Organs. —These are represented by tactile organs l auditory organs (otoliths), and eyes. The whole surface of the body is very sensitive and (e.g., in the Polycladida) contains 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 Rhabdocoela 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 innervated, 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 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 processes on the summit of the dorsal papillae and the various tentacular structures; the tentacles in this family also serve to support the eyes.

The majority of the Turbellarians possess eyes; the Rhabdocoelida 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 Polycladida 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 situated 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 Carrière 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 Rhabdocoelida (Acoela, Monotida, fig. 4, ot) embedded in a depression on the anterior surface of the brain. In the Dendrocoelida these organs are but rarely present.

As a sensory organ of unknown function must be mentioned the paired lateral ciliated grooves which are met with on either side of the brain in many Rhabdo oela (fig. 9, c); they are also found commonly in Nemertines (q. υ.), 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 Microstonιida 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 surface (most Rhabdocoelida 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- coelida and all Polycladida, fig. 8), but occasionally the positions are reversed (certain Rhabdocoelida). The genital glands display a primitive condition in being paired, though frequently the germarium (fig. 5, g) of the Rhabdocoela, 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 {oυ 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 germarium (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 uteras 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 additional 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 Rhabdocoel Turbellarians {e.g., Mesostoma 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 bursae seminales, which receive the semen during copulation and retain it...