228 HYDRA 



The endoderm cells (A and B, end.} are of two kinds- 

 larger and smaller. The larger cells exceed in size those 

 of the ectoderm, and are remarkable for containing one 

 or more vacuoles, sometimes so large as to reduce the 

 protoplasm to a thin superficial layer containing the 

 nucleus. 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 trans- 

 verse 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 referred to (52). 



In the hypostome the endoderm is thrown into longitu- 

 dinal folds, so as to allow of the dilatation of the mouth in 

 swallowing. The smaller cells (A) are long and narrow, and 

 have the character of gland-cells : the secretion in this case 

 is probably 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. In H. fusca bodies 

 resembling these chromatophores are present, but are of an 

 orange or brown colour, and devoid of chlorophyll. Brown 

 and black granules occurring in the cells (B) seem to be due, 

 in great measure at least, to the degeneration of the 

 chromatophores. 



Muscle-processes exist in connection with the endoderm 

 cells, and they are said to take a transverse or circular 

 direction, /.., at right angles to the similar processes of 

 the ectoderm cells. 



