ON THE STEUOTURE OE HYDRAOTINIA ECHINATA. 95 



111 a gasterozooid which has had no food for some time the 

 protoplasm of the eudoderm cells is exceedingly vacuolated, 

 there being only a small mass of protoplasm congregated round 

 the nucleus, and a thin protoplasmic cell lining; no other 

 endodermal elements can be demonstrated, at all events in 

 dead material. 



On the other hand, an examination of the endoderm of a 

 gasterozooid which is digesting food clearly shows that there 

 is a certain amount of differentiation between the cells. The 

 ordinary vacuolate endoderm cells are crowded with food 

 vacuoles; the nuclear plasm appears more granular than in 

 non-digesting endoderm cells. But besides these vacuolate 

 cells, there are numbers of pyriform cells which are situated 

 singly between every two or three of the vacuolate cells, and 

 which stain very readily. These cells are rather wider in 

 their middle region than the vacuolate cells, and taper off 

 basally where they come in contact with the mesogloea ; the 

 nucleus is situated towards the base of the cell and is sur- 

 rounded by a dense mass of protoplasm. The rest of the ceil is 

 crowded with small spherical bodies, which are so abundant as 

 to hide the protoplasm in which they are embedded (PI. l,fig.9). 



Miss Greenwood (16) describes similar cells in the endoderm 

 of Hydra under the name of "^ gland-cells.'^ Among the cell 

 contents of the endoderm are numerous roundish bodies, 

 smaller than nuclei, and especially abundant in the endoderm 

 of the hypostome ; some of these stain equally throughout, 

 but others only stain partially, a large slightly stained 

 region being left. They stain deep blue with haematoxylin 

 but yellow with borax carmine ; they are colourless in the 

 living animal, and stain with iodine less than the other cell 

 contents. The most likely hypothesis seems to be that they 

 are proteid in nature ; they may correspond to the " nutritive 

 spheres" of Hydra described by Miss Greenwood (16). Endo- 

 derm is continued into the tentacles as a single row of cells 

 which are very vacuolated and regular in shape; the nucleus 

 is central and surrounded by protoplasm^ which forms a central 

 band in the cell. 



