608 SHEINA MARSHALL 



1 9 /i) , t he penetrants now measured only 1 1 -1 5 /it. This was due, 

 in the first place, to the disappearance of the large type. The 

 other nematocysts, however, also showed a similar though 

 slighter decrease in size. The same experiment with another 

 Hydra (starved forty-seven days) yielded similar results. 

 If the size of the nematocysts varies with the size or condition 

 of the individual it is obvious that this cannot be used as 

 a trustworthy characteristic in differentiating between the 

 various species. 



In Hydra viridis the penetrants measure 8-1 /^ ; the 

 streptoline glutinants are kidney-shaped and are as large 

 as the penetrants, being 10-11 /x. The stereoline glutinants 

 are much smaller and rarer, being only 6-7 /x, and the volvents 

 are about 5fi. 



To obtain the nematocysts freely the Hydra was at first 

 macerated in weak chloroform water. Latterly I used Schulze's 

 phenol-glycerine mixture (phenol, crystallized, 1 grm., glycerine 

 200 c.c, distilled water 200 c.c), as giving the same results 

 and being more convenient to handle. 



Nervous System. 



The distribution of the nervous elements is such as one 

 would expect from the reactions of the living animal. They 

 are most numerous in the foot and in the head and tentacles, 

 and are much scarcer in the middle part of the body. 



The nervous cells are best seen in maceration preparations. 

 I have found the most useful method to be maceration in 

 Hertwig-Schneider's mixture (0-02 per cent, osmic acid, 

 1 part : 5 per cent, acetic acid, 4 parts) for fifteen to twenty 

 minutes and subsequent staining in a strong filtered solution 

 of methylene blue for three-quarters of an hour or longer. 

 The methylene blue is dissolved until a deep blue solution is 

 obtained. The specimen is then well macerated and several 

 gentle taps on the coverslip are sufficient to separate the 

 cells. By dividing the Hydra before maceration has begun 

 a fair idea of the relative distribution of the nervous cells may 

 be obtained. 



