78 



PLANARIANS 



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 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 ofE 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 

 epidermis is a basement membrane (fig. 2, hm) which is in 



Fio, 2. — Integument of Mesostoma lingua, O. Sch. On the light hand is the 

 epidermis (z) with perforations (I) through which the rhabdites (st) project. 

 Beneath tliis the basement membrane (6m), and beneatli this again the muscular 

 layers consisting of circular Cm), diagonal (sm), and longitudinal (Im) 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 

 {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 (Dendrocoslida) 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 trabeculse, 

 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 tbere 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, «). The intestine (i) has a very 

 characteristic form in the different sections, and has long 

 served to divide the Turhellaria into two groups : — (1^ 

 Rhabdocoelida, with a straight unbrariched intestine (figs. 5, 

 6), and (2) Dendrocoslida, 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 " stomacJT," 

 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 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 Acoela to the 

 lowest position not only among the Turhellaria, 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 



