606 SHEINA MARSHALL 



Schulze, H. stellata, Schulze, and Pelmatohydra 

 b r a u e r i , Schulze) the streptoline is not cyHndrical but pear- 

 shaped. When Hydra sticks on to glass or to any other 

 surface by means of its tentacles or hypostome, it uses these 

 nematocysts. If one of the adherent tentacles is examined, 

 there are seen numerous exploded glutinants whose threads 

 are firmly attached to the glass. They are so firmly fixed that 

 if any pull is exerted on them the cell protoplasm of the 

 Hydra is drawn out into a thread with the nematocyst at 

 its tip. Zygoff (16) was the first to notice these, and looked on 

 them as pseudopodia by which the animal moved. Toppe 

 discovered their true nature. These processes can withstand 

 a considerable strain. I have seen a Hydra apparently 

 trying to free a tentacle which was held at the tip by one 

 of these nematocysts only. The tentacle was given several 

 tugs, was twirled round rapidly, first in one direction and then 

 in the other, the animal contracted tightly once or twice, and 

 finally the tentacle was torn away. It w^as striking to watch 

 an animal like the Hydra exhibiting such apparently pur- 

 poseful movements. The twirling movements are much more 

 complex than any the Hydra usually shows, and must have 

 called into play a different mechanism. The nematocyst is 

 always left sticking to the substratum while the protoplasmic 

 process is gradually withdrawn into the cell. A similar process 

 is sometimes drawn out when a tentacle is pulled away from 

 some bristle on which a volvent has wound itself. 



In unfixed, exploded glutinants stained with methylene 

 blue, numerous droplets can be seen on the outside of the 

 thread, as in the case of the penetrants. This secretion is 

 probably sticky, and possibly hardens in contact with water. 

 When used, it is extruded not only by the pore at the end of 

 the thread but also by the side pores, for the thread can often 

 be seen adhering at a point about half-way down its course. 



It seems probable that the secretion of the other types of 

 nematocyst has also to some degree the property of sticking 

 firmly. I have observed tentacles adhering both by the 

 penetrants and by the volvents, though not so firmly. 



