Chap, iv.] DIGESTION IN METAZOA. 113 



proteids by the method of in trace! lular digestion, 

 and they appear to be the only part of the organism 

 which is entrusted with this duty. 



The Ctenophora have the spaces in connection 

 with the axial gastric cavity narrowed to four canals, 

 and there are two pores at the aboral pole of the body. 

 There are never more than two long tentacles, and 

 when these are lost, as in Beroe, the mouth is much 

 wider than in the tentaculate forms. 



For the rest of the Metazoa, with the exception of 

 the already mentioned Turbellaria and Trematoda 

 (e.g. liver fluke), the intracellular mode of digestion 

 has not been observed. As in some of the Coelenterata, 

 we have a higher mode ; the cells of the endodermal 

 lining of the gastric tube have now ceased to act in- 

 dependently of one another; certain of them are 

 set apart for the function of secreting a ferment, 

 which, passing from them into the digestive cavity, 

 there acts upon the food ; the albuminoids contained 

 in it are converted into substances capable of passing 

 through the wall of the intestine. Special salivary 

 glands are, in many, developed for the purpose of con- 

 verting starch into sugar. There is some evidence, 

 however, that certain cells continue to take up nutri- 

 ment into their own substance ; even in the frog some 

 of the cells of the small intestine have been observed 

 to send out short processes into the enteric cavity 

 (Thanhoffer), recalling thereby the amoeboid cells and 

 the intracellular mode of digestion which is seen in 

 Hydra. 



Among the flat-worms we need here only consider 

 the Turbellaria and flukes, as the tapeworms obtain 

 their nutriment in a very special way. (See page 177 ; 

 Digestion of parasitic animals.) A mouth is always 

 present, but is by no means constant in position, as 

 it may be far forwards, at the middle of the body, 

 or far back (Opisthomum). In a number there is 

 116 



