286 



HISTOLOGY 



cretion granules ; f.v., food vacuoles; 

 nu nucleus x 870. (From a prepa- 

 ration by A. H. TUTTLE.) 



In this way the currents of water con- 

 tinually sweep by the alimentary tissue, 

 t^'rs;x^v;;.^^^-^"x^:\ preventing digestive secretions from act- 



W^I^S^v^ljfc 3JJJ m & u P on f 00 ^ that ma y loc *g e outs ide 

 ^l^^^f&MI^^ of the alimentary cells. The food must 



r.'Y'x '' -^V ''';8Sg';"i '''/ 



F : ^;;-V-r^ ; :.>::^^V^;:y therefore be taken into the cell bodies 



l^^nV^&^J^^^y/ to be digested within food vacuoles ; com- 



paratively large bodies are taken into 

 these food vacuoles (see Fig. 246). 



An example of simple infra-cellular 

 alimentary tissue. In the Ccelenterata 



J* J 'l^ i W; : l^^~' w ^' } sec - 9- and Platyhelminthes the endoderm has 

 'IllSS-^^iiy / 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 



rents of water. Here we meet with, in 

 our study, the first cavity that is to 

 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 from 



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- 





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. 



