NUTRITION 



that surrounds the mouth), it is bilateral-symmetrical in 

 the platodes or "flat-worms" (plathelminthes). In this 

 animal-stem, moreover, the lowest forms, the platodaria 

 (also called cryptoccela and accsla) come very close to the 

 gastrsea. But most of the platodes are distinguished 

 from the rest of the coelenteria by the formation of a pair 

 of nephridia (renal canals or water- vessels), thin tubes 

 which, as excretory organs, remove from the body the 

 unusable products of metabolism, the urine. Here we 

 have a second organ of nutrition, the gut tube, added to 

 the first. In the lower platodes this remains very 

 simple. As a rule, a gullet tube (pharynx) is formed by 

 the hollowing out of the mouth, as in the corals; and as 

 in the case of the latter branched canals, which conduct 

 the nutritive sap from the stomach to distant parts of the 

 body, grow out of the stomach, in the larger coil-worms 

 (turbellaria) and suction-worms (trematodes). On the 

 other hand, the gut atrophies in the tape-worms (ces- 

 todes) ; as these parasites live in the intestines or other 

 organs of animals, they can obtain their nutritive sap 

 directly from them through the surface of the skin. 



The more highly organized coelomaria differ from the 

 simpler coelenteria chiefly by the greater complexity in 

 the structure and functions of their apparatus of nu- 

 trition. As a rule, these functions are divided between 

 four groups of organs, which are not yet differentiated 

 in the coelenteria — namely: i, organs of digestion 

 (gastric system); 2, organs of circulation (vascular 

 system); 3, organs of breathing (respiratory system); 

 and 4, organs of excretion (renal system). Moreover, 

 in the coelomaria the gastric canal has usually two 

 openings, the mouth and the anus. Finally, they all 

 have a special body-cavity {cceloma) ; this is quite separate 

 from the gastric canal, which is suspended in it, and 

 serves for the formation of the sexual cells. It is 

 formed in the embryo by the hollowing out and cutting 

 IS 225 



