158 



COMPARATIVE ANATOMY. 



the edge of the body ; au elegant mesh-work may be formed by 

 the connections of these with one another (Thysanozoon). Owing 



to the free communication that these 

 branches have with the central cavity, 

 the chyme is distributed in the body, and 

 so the enteric canal takes on the function 

 of a vascular system. The land Plana- 

 rians are remarkable, inasmuch as their 

 enteric tube consists anteriorly of a median 

 canal, while it is divided posteriorly. Nu- 

 merous and regular transverse processes 

 pass off from both divisions of it. 



The enteric tube is branched in many 

 of the Trematoda. The gut commences 

 by a mouth, which is generally placed 

 in the anterior region of the body, and, 

 as a rule, has the surrounding parts meta- 

 morphosed into a sucker (Fig. 69, s) ; this 

 is followed by a muscular pharynx (b), 

 from which the enteron proper is given 

 off. This is, when most simple, a ceecal 

 sac (Aspidogaster, Gasterostomum), and 

 corresponds to a low grade of develop- 

 ment; this is very common among the 

 Trematoda at certain stages of their de- 

 When more differentiated, the enteron 

 divides into two branches, which pass backwards, and either give 



off greatly ramified branches into the body 

 (Distoma hepaticum), or form simple 

 cascal sacs (c) (Distoma flavescens, D. 

 lanceolatum) . The two branches may 

 unite again and form an arrangement like 

 that which obtains in some Planarise. It 

 is clear, from the homogeneity of its struc- 

 ture, as well as from its contents, that 

 even in the Trematoda this branching of 

 the gut is merely an enlargement of the 

 tract in the body, and not the formation 

 of heteronomous segments. The texture 

 of the wall is in correspondence with the 

 low stage of this form of enteron, for 

 only the epithelial investment is indepen- 

 dent, being bounded, exteriorly, by the 

 tissue of the parenchyma of the body — 

 connective tissue. 



Complete degeneration of the gut is 



clearly due to adaptations to definite 



modes of life, in which the food passes through the integument 



by cndosmosis. This phenomenon, brought about by parasitism, 



Fig. 68. Digestive apparatus 

 of Eurylepta sangui no- 

 lent a. o Mouth. $> Pharynx. 

 v Stomach. gv Ramifica- 

 tions of the digestive cavity. 

 n Nerve ganglion (brain) (after 

 Qnatref ages) . 



velopment (Redia-f orin) . 



Fig. G9. Alimentary canal 

 of Distoma flavescens. 

 o Mouth surrounded by a 

 sucker, s. s' Ventral sucker. 

 b Muscular portion of the oeso- 

 phagus or pharynx.- c Bifur- 

 cated enteric tube. 



