Ill 



PLATODES ORGANS OF LOCOMOTION 



143 



In the so-called Accela the alimentary canal is represented by a 

 mass of star-shaped branched cells, in which no intestinal cavity can 

 be demonstrated. Food enters this mass direct through the mouth or 

 oesophagus ; it has been described as "digesting parenchyma," 



Trematoda. The gastro-canal system of the Trematoda most 

 resembles that of the Triclada. Since the 

 mouth and pharynx lie to the front, the 

 intestinal aperture leads into the anterior 

 end of a short unpaired median portion of 

 the intestine called the oesophagus ; this 

 soon splits into two forked branches which 

 run backward (Fig. 106). In the broad leaf- 

 shaped Trematoda to which most of the 

 ectoparasitic forms and also the liver fluke be- 

 long, lateral branches, which again divide, run 

 mostly outwards from these forked branches. 



In certain forms (Stichocotyle, Aspidogaster) 

 the intestine runs back as an unpaired 

 median caecum. When a median sucker is 

 developed at the anterior end (oral sucker) 

 the mouth lies in its base. 



The intestinal epithelium of the Turbellaria is 

 covered, over larger or smaller areas, with cilia. 

 Intracellular taking in of food is very common 

 in the Turbellaria. The musculature of the gastro- 

 canal system is on the whole very feebly developed, 

 and consists of longitudinal and circular fibres. In 

 the Polydada single circular muscle fibres succeed 

 each other on the intestinal branches at regular 

 intervals, so that in consequence of the constrictions 

 thus formed the intestinal branches assume the aspect 

 of strings of beads. 



As an anus is wanting, the ftecal masses are emptied 

 out through the mouth. The gastro-canal system of 

 the Turbellaria may, besides the functions of diges- 

 tion and circulation, also have a respiratory signifi- 

 cance. 



FIG. 106. Intestinal and nerv- 

 ous systems of Distoma isosto- 

 mum (Trematode), after Gaffron. 

 ms, 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 Organs 

 of 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 and 

 tolerably firm and elastic, plays the part of a supporting membrane, 

 to which the dermal musculature is closely applied, and in which 

 the dorso- ventral muscle fibres are inserted. The same part is also 

 certainly played by the cuticle of the Trematoda and Cestoda. Besides 

 this, the more or less compact parenchyma lends the body a firmer 



