8o PHYSIOLOGY OF THE INVERTEBRATA. 



enclosed food particles takes place entirely outside the endo- 

 dermic cells which line the enteric cavity, and among these 

 may be distinguished: (i) pyriform cells destitute of large 

 vacuoles holding secretory spherules daring hunger, and 

 these empty during digestive activity ; (2) ciliated vacuolate 

 cells, often pigmented : the water of the digestive fluid is 

 probably derived from the vacuoles, (c) The pigment occurs 

 as brown or black grains ; it has an albuminoid basis. The 

 pigment resists solution in most chemical reagents, but 

 dissolves slowly in nitric acid. (S) A reserve substance of an 

 albuminoid nature accumulates during digestion in the basal 

 part of the vacuolated cells, and eventually takes the form of 

 spheres. The excretory pigment probably takes its rise in 

 some residue from this absorbed substance ; it is also possible 

 that fat is similarly formed, (e) The medium in which 

 digestive activity goes on is probably not acid. 



In Hydra viridis, which contains chlorophyll,* the mode of 

 nutrition appears to be difEexent from that just described...!; 

 Gland cells do not form a conspicuous feature in the endo- 

 derm of Hydra viridis, and consequently digestive secretion 

 is less active. 



If the vacuolated cells of the endoderm of Hydra fusca 

 contain a nutritive fluid we may reason, d priori, that the 

 food vacuoles of the Protozoa probably contain a digestive 

 fluid; at any rate the food particles nearest to these vacuoles 

 are always becoming smaller in size, showing that digestion is 

 proceeding. 



In Hydra viridis chlorophyll has probably a secretory as 

 well as a respiratory function. The same remark applies to 

 ■the chlorophyllogenous Protozoa. "Professor Huxley first 

 showed the presence of " yellow cells " in Thalassicolla, which 

 have also been found in almost all Eadiolarians. These 

 bodies Haeckelf considered as secreting cells or digestive 

 glands, comparable to the liver cells of Amphioxus, and those 



* The chloroplastids of Prof. E. Eay Lankester, F.R.S. 

 t Die Eadiolarien, p. 136. 



