DIGESTIVE SYSTEM 119 



The fundamental form is the cell with its protoplasm pos- 

 sessing ingestive and egestive powers. The lowest of the pro- 

 tozoa have no distinct organ for the reception and digestion 

 of food. The food is either admitted endosmotically, in fluid 

 form (gregarina), or enclosed in solid form by any part of the 

 non-differentiated body and digested, the undigested matter 

 being discharged at any point of the body (monera, amoeba, 

 rhizopoda). 



The infusoria possess distinct organs for the reception of 

 food in the form of tubular processes radiating from the sur- 

 face of the body ; indeed a mouth and anus may be said to be 

 present but without an intestinal tube. 



The porifera are furnished with a system of canals having 

 mouthlike orifices not only for the admission and rejection of 

 water, but also for the reception of food. The ccelenterata 

 possess a digesting cavity in the middle of the body with a 

 ciliated orifice opening outward ; this structure may be said to 

 constitute a stomach surrounded by parenchyma. 



Combined with this primitive stomach in the majority of 

 the ccelenterata, are canalicular or pouch-like formations, but an 

 anal orifice opposite the mouth does not exist, the orifice of the 

 mouth serving also for the discharge of undigested matter. 



Signs of considerable progress in the development of the 

 digestive organ are seen in the worms, whose digestive ap- 

 paratus is either imbedded in the parenchyma, or situated in 

 the abdominal cavity. The structural improvement consists 

 not only in the fact that, in most cases, in addition to the 

 mouth at one end of the body, a dorsal or ventral anal orifice 

 is present at the opposite extremity, but in the far more 

 significant fact that three distinct intestinal segments are to be 

 distinguished, namely, the small, the large, and the terminal 

 intestine. Indeed, in the tunicata we find an oral cavity, in 

 the form of the sac-like anterior part of the body, and a dis- 

 tinct expansion of the large intestine. 



In the echinodermata, even in their larval state, a further 

 development of the intestinal differentiation which commences 

 in the worms may be recognised. The circumstance that the 

 oral and anal orifices, originally situated diametrically opposite 

 each other, eventually lie on the same surface is no essential 



