xxv DIGESTION 281 



segment ; it is laterally compressed so as to have an 

 elongated form in cross section (c, and Fig. 71, A) : the 

 fourth portion or rectum (Ret) is confined to the anal seg- 

 ment ; it is somewhat dilated and is not laterally compressed. 

 The epithelium of the intestine is, as indicated in the 

 diagram (B), endodermal ; that of the remaining division of 

 the canal is ectodermal. The large majority of the cells in 

 all parts of the canal are ciliated. 



The cells of the enteric canal and especially those of the 

 gullet are very granular, and like the endoderm cells of the 

 hypostome of Hydra (p. 231) are to be considered as gland 

 cells. They doubtless secrete a digestive juice which, 

 mixing with the various substances taken in by the mouth, 

 dissolves the proteids and other digestible parts, so as to 

 allow of their absorption. There is no evidence of intra- 

 cellular digestion such as occurs in Hydra (p. 232), and it is 

 very probable that the process is purely extra-cellular or 

 enteric, the food being dissolved and rendered diffusible 

 entirely in the cavity of the canal. By the movements of 

 the canal caused partly by the general movements of the 

 body and partly by the contraction of the muscles of the 

 septa aided by the action of the cilia, the contents are 

 gradually forced backwards and the sand and other indi- 

 gestible matters are expelled at the anus. 



The coelome is filled with a colourless transparent 

 cceloniic fluid in which are suspended minute, irregular, 

 colourless bodies, as well as oval bodies containing yellow 

 granules. From the analogy of the higher animals one 

 would expect these to be leucocytes (p. 56), but their 

 cellular nature has not been proved. 



The function of the ccelomic fluid is probably to distribute 

 the digested food in the enteric canal to all parts of the 



