280 COMPARATIVE PHYSIOLOGY. 



and hypoWast, it is easy to understand why^ the developed tract 

 should so invariaably consist of glandular structures and mus- 

 cular tissue disposed in a certain regular arrangement. The 

 fact that all the organs that pour digestive juices into the ali- 

 mentary tract are oatgrowths from it serves to explain why 

 there should remain a physiological connection with an ana- 

 tomical isolation. The general resemblance of the epithelium 

 throughout, even in parts widely separated, also becomes clear, 

 as well as many other, points we can not now refer to in detail, 

 to one who realizes the significance of the laws of descent (evo- 

 lution). 



Comparative. — Amoeba ingests and digests apparently by 

 every part of its body ; though exact studies have shown that 

 it neither accepts nor retains without considerable power of 

 discrimination ; and it is also possible that some sort of digest- 

 ive fluid may be secreted from the part of the body with which 

 the food-particles come in contact. It has been shown, too, 

 that there are differences in the digestive capacity of closely 

 allied forms among Inf usorians. 



The ciliated Infusorians have a permanent mouth, which 



may also serve as an anus ; or, there may be an anus, though 



usually less distinct from the rest of the body than the mouth. 



/ Among the Coelenterates intra-cellular digestion is found. 



Certain cells of the endoderm (as in Hydra) take up f ood-parti- 



Icles Amoeba-like, digest them, and thus provide material for 



jother cells as well as themselves, in a form suitable for assimi- 



llation. This is a beginning of that differentiation of function 



I which is carried so far among the higher vertebrates. But, as 



I recent investigations have shown, such intra-cellular digestion 



, exists to some extent in the alimentary canal of the highest 



/ members of the vertebrate group (see page 345). 



The means for grasping and triturating food among in- 

 vertebrates are very complicated and varied, as are also those 

 adapted for sucking the juices of prey. Examples to hand are 

 to be found in the crab, crayfish, spider, grasshopper, beetle, 

 etc., on the one hand, and the butterfly, housefly, leech, etc., on 

 the other. 



Before passing on to higher groups, it will be well to bear 

 in mind that the digestive organs are to be regarded as the out- 

 come both of heredity and adaptation to circumstances. "We 

 find parts of the intestine, e. g., retained in some animals in 

 I whose economy they seem to serve little if any good purpose, as 



