28O HISTOLOGY 



of the cell. This is the fundamental, structural device by which all 

 intra-cellular alimentation is accomplished. 



In all Metazoa the alimentary tissue is differentiated and distinct 

 from the other tissues of the body; and in all cases it is an epithelial 

 tissue. Despite the presence of a distinct alimentary tissue, however, 

 and until a definite food cavity or enteron is established, pseudopods, 

 flagella, cilia, and food vacuoles are the only structural, alimentary 

 features. The endoderm of the Porifera or sponges is an alimentary 

 tissue composed of a simple collared epithelium, each cell of which is 

 provided with a flagellum. This epithelium lines a cavity which is not 

 an efficient place for digesting fluids to act upon food, because of the 

 currents of water that constantly stream in and out of it, and which would 

 carry away the secretions. Food, therefore, is thrown into the cell 

 body of an endodermal cell by means of the flagellum, and alimentation 

 takes place within a food vacuole in a manner analogous to the process 

 used by the Infusorian. 



The food vacuole limits the size of the particles of food consumed and 

 is, therefore, not an highly efficient alimentary structure. With the 

 progressive development of the food cavity or enteron, the food vacuole 

 becomes less frequent. 



In the Ccelenterata, polyps, and jellyfish, a food cavity or enteron 

 is formed which is open at but one region, and thus supplies a place for 

 digestive fluids to act upon food outside of the cell body. Here pseu- 

 dopods and cilia attend both food vacuoles and an enteron. Besides 

 the cells, bearing food vacuoles, there are others which elaborate di- 

 gestive fluids to be discharged upon the food and to digest it in the 

 enteron. 



Before the enteron becomes an open tube, it is supplied with a mus- 

 cular coat which, by a churning action, aids the extra-cellular alimenta- 

 tion. In some of the platyhelminthes the endodermal cells may bear 

 food vacuoles. The simple columnar epithelium, however, is an ali- 

 mentary tissue acting chiefly upon food contained within the lumen 

 of the enteron. Here there is a slightly evident differentiation between 

 cells of digestion and cells of absorption. But the cells so differentiated 

 are not assembled to form two tissues. 



In the Annelida and all higher forms, the enteron is made more 

 efficient by a stomodaeum and a proctodaeum. With this advance 

 in the formation of the general food cavity, there arises a differentia- 

 tion of alimentary tissues into digestive and absorptive tissues. 



This differentiation together with the development of a separate 

 internal digestive cavity enables the organism to use a great variety of 

 foods, some of which are bulky in nature or hard or tough. Such food 

 must be first mechanically cut off to be swallowed, and some of it ground 



