286 



HISTOLOGY 



In this way the currents of water con- 

 tinually sweep by the alimentary tissue, 

 preventing digestive secretions from act- 

 ing upon food that may lodge outside 

 of the alimentary cells. The food must 

 therefore be taken into the cell bodies 

 to be digested within food vacuoles ; com- 

 paratively large bodies are taken into 

 these food vacuoles (see Fig. 246). 



An example of simple intra-cellular 

 alimentary tissue. In the Ccelenterata 

 ~/nu. ^.sec. g. and Platyhelminthes the endoderm has 

 been differentiated to perform the func- 

 tion of alimentation. It is of interest 

 here to note that the alimentary tissue 

 lines a cavity which is open at but one 

 end. This affords a place for food to 



retion granules; f.v., food vacuoles; rents of water. Here we meet with, in 



any degree efficient as an extra-cellular 

 cavity or enteron. The cells lining this cavity are all of tall co- 

 lumnar forms. The oval nuclei lie near 

 the base or in the lower third of all the 

 cells. The distal ends of the cells are ex- 

 panded and may bear many vacuoles. 

 There is here an interesting differentiation 

 of the alimentary cells into two types of 

 cells : the ordinary cells which we may call 

 the absorptive cells, and the albumen cells 

 which elaborate a digestive ferment; hence 

 we call them the digestive cells (Figs. 248 

 and 249). The cytoplasm of the albumen 

 or digestive cells is much denser than that 

 of the absorbing cells. They are usually 

 shorter than the absorbing cells. Their 

 secretion products are elaborated in the 



form of spherical bodies at the proximal FIG. 249 . Six digestive cells f 

 end. The nuclei are easily distinguished 

 from those of the connective-tissue cells, 

 but the nuclei of digestive and absorbing 

 cells cannot be distinguished. It is inter- 

 esting to note that both in Hydra and Bdel- 



rom 



the enteron of Bdellura Candida. 

 They show some food vacuoles 

 and secretion. In both this and 

 the preceding example the dark 

 cell probably secretes a different 

 ferment from that produced by 

 the lighter cells. X 870. 



