Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/181

Rh 171 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 Culentera (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 &quot;glutinous cells,&quot; 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 epidermis is a basement membrane (fig. 2, bin) which is in bm. J | SS@W&amp;lt;vJ^. &quot; -.. &quot; : -:~ .- &quot; ,:- :. v 1 1 : -Wr-U &quot;^ ^&quot; -Ki-*v;f;^ hit Fir.. 2. Integument of Mesostonta lingua, O. Sch. On the right hand is the epidermis (z) with perforations (/) through which the rhabdites (st) project. Beneath this the basement membrane (bin), and beneath this again the muscular layers consisting of circular (.nn). diagonal (SHI), and longitudinal (Jm) fibres. 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 inter posed. The larger forms (Dendrocoelida) 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 trabecuke, 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. Ml 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, pit) is cylindrical in form, rather complicated in structure, and surrounded by a muscular sheath, which opens on to the exterior by the mouth (in}. 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 &quot;stomach,&quot; 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 net work. The epithelium of the intestine is a single layer of cells generally not ciliated, capable of protruding amujboid 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 Acacia 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