Text-book of comparative anatomy . hes, which again divide, runmostly outwards from these forked branches. In certain forms (Stichocotyle, Aspidogaster)the intestine runs back as an unpairedmedian caecum. When a median sucker isdeveloped at the anterior end (oral sucker)the mouth lies in its base. The intestinal epithelium of the Turbellaria iscovered, over larger or smaller areas, with taking in of food is very commonin the Turbellaria. The musculature of the gastro-canal system is on the whole very feebly developed,and consists of longitudinal and circular fibres. Inthe P
Text-book of comparative anatomy . hes, which again divide, runmostly outwards from these forked branches. In certain forms (Stichocotyle, Aspidogaster)the intestine runs back as an unpairedmedian caecum. When a median sucker isdeveloped at the anterior end (oral sucker)the mouth lies in its base. The intestinal epithelium of the Turbellaria iscovered, over larger or smaller areas, with taking in of food is very commonin the Turbellaria. The musculature of the gastro-canal system is on the whole very feebly developed,and consists of longitudinal and circular fibres. Inthe Polydada single circular muscle fibres succeedeach other on the intestinal branches at regularintervals, so that in consequence of the constrictionsthus formed the intestinal branches assume the aspectof strings of beads. As an anus is wanting, the ftecal masses are emptiedout through the mouth. The gastro-canal system ofthe Turbellaria may, besides the functions of diges-tion and circulation, also have a respiratory FIG. 106.—Intestinal and nerv-ous systems of Distoma isosto-mum (Trematode), after , Oral sucker; ph, pharynx;gel, forked branches of the intesti-nal canal; gc, cerebral commissure;dn, dorsal longitudinal nerves ; sn,lateral longitudinal nerves; en,ventral longitudinal nerves; 6s,ventral sucker. V. Supporting Organs, Passive Organsof Locomotion. These are in most soft and delicate Pla-todes little developed. In the Turbellaria, at least in the Polydada, the basal membrane, which is resistant andtolerably firm and elastic, plays the part of a supporting membrane,to which the dermal musculature is closely applied, and in whichthe dorso-ventral muscle fibres are inserted. The same part is alsocertainly played by the cuticle of the Trematoda and Cestoda. Besidesthis, the more or less compact parenchyma lends the body a firmer 144 COMPARATIVE ANATOMY CHAP. structure and a greater consistency, like the gelatinous tissue inthe Cnidaria. VI. The
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