122 BIOLOGY: GENERAL AND MEDICAL 



hydra where a well-formed digestive cavity is present. 

 Large objects entering the oral orifice are dissolved in 

 the gastric cavity by enzymes derived from the ento- 

 dermal cells, and the assimilable products of this diges- 

 tion are absorbed. Small particles are liable to be 

 seized upon by the cells and treated by the more primi- 

 tive process of phagocytosis or intracellular digestion. 

 The more complex ccelenterates continue to display 

 more or less of this phagocytosis of the entodermal 

 cells, but as we ascend into the phylum Annulata the 

 specialization of the entodermal cells as digestive enzyme 

 producers becomes so distinct that it disappears not to 

 be seen again among the higher animals. 



The primitive digestive system exemplified by the 

 gastric cavity of hydra, in which the oral orifice sub- 

 serves the double purpose of mouth and anus, finds an 

 improvement in the echinoderms and worms by the 

 addition of a separate anus at its aboral end. The food 

 ingested now passes slowly through the enteron as 

 Haeckel has called this primitive stomach-intestine, 

 being digested, during its passage, the residuum being 

 discharged from the anus. The further specializations 

 are for the most part in the improvement and localiza- 

 tion of the digestive forces. The enteron becomes 

 more and more tubular and gradually separates itself 

 into a mouth for mastication, which is provided with a 

 variety of different appendages, teeth for crushing and 

 comminuting, salivary glands for moistening the food, 

 etc., a gullet through, which the food enters the main 

 digestive viscus, a stomach, and an elongated intestine 

 from which absorption of the products of digestion may 

 take place. 



Instead of the digestion being partly intracellular, 

 the cells are specialized as enzyme producers, arranged 

 in glands by which digestive juices are poured into the 

 alimentary canal and mixed with the food so that com- 

 bined maceration, solution, and digestion may prepare the 

 assimilable substances for absorption from the intestine. 



