ON THE STRUCTURE OF HYDRAOTINIA EOHINATA. 



93 



These cells are apparently not characterised by muscle tails, 

 but interstitial cells are sometimes wedged in between them 

 at their bases (PI. 1, fig. 5). There are no sensory cells in 

 this layer, but nematocysts are occasionally found. The nucleus 

 is situated in the centre of the cell ; it is round or slightly oval, 

 and has always one large nucleolus : surrounding it is a mass 

 of finely granular protoplasm which almost fills the whole cell. 



2. Ectoderm of the Polyps. — This layer is in many 

 respects similar to the basal ectoderm ; the cells are often 

 somewhat narrower, more columnar in shape, and more variable 

 in size. 



They are characterised by exceedingly long muscle tails, 

 which can be demonstrated by macerating a hydroid for about 

 an hour in i per cent, solution of acetic acid, washing in water, 

 staining with picro-carmiue for an hour or more, and examin- 

 ing in weak glycerine. 



The protoplasm of the muscle tails appears more homoge- 

 neous and less coarsely granular than that of the body of the 

 cell. In several of these isolated ectoderm cells large vacuoles 

 were observed. (Process, Fig. 3.) 



Fig. 3. 



