xxii ENDODERM 231 



and the cell performing the function is known as a gland 

 cell. 



The endoderm consists for the most part of large cells 

 which exceed in size those of the ectoderm, and are re- 

 markable for containing one or more vacuoles, sometimes 

 so large as to reduce the protoplasm to a thin superficial 

 layer containing the nucleus (Fig. 51, A and B, end}. Then 

 again, their form is extremely variable, their free or inner 

 ends undergoing continual changes of form. This can be 

 easily made out by cutting transverse sections of a living 

 Hydra, when the endoderm cells are seen to send out long 

 blunt pseudopods (psd] into the digestive cavity, and now 

 and then to withdraw the pseudopods and send out from 

 one to three long delicate flagella (fl\ Thus the endoderm 

 cells of Hydra illustrate in a very instructive manner the 

 essential similarity of flagella and pseudopods already re- 

 ferred to (p. 51). In the hypostome the endoderm is thrown 

 into longitudinal folds, so as to allow of the dilatation of 

 the mouth in swallowing. 



Amongst the ordinary endoderm-cells are found long 

 narrow cells of an extremely granular character. They are 

 specially abundant in the distal part of the body, beneath 

 the origins of the tentacles, and in the hypostome, but are 

 absent in the tentacles and in the foot. There is no doubt 

 that they are gland-cells, their secretion being a fluid used 

 to aid in the digestion of the food. 



In Hydra viridis the endoderm-cells (D) contain chroma- 

 tophores (chr) coloured green by chlorophyll, which performs 

 the same function as in plants, so that in this species holozoic 

 is supplemented by holophytic nutrition. There is reason 

 for believing that the chromatophores are to be regarded as 

 symbiotic algce, like those found in connection with Radio- 



