604 



SHEINA MARSHALL 



The large pear-shaped nematocyst is, as the name penetrant 

 implies, used for piercing the chitinous exoskeleton of the 

 prey. When the hard shell has been pierced, the thread is 

 everted into the soft tissues beneath. Usually only that part 

 of the nematocyst immediately above the large barbs succeeds 

 in penetrating, but as the thread itself has a spiral coil of small 

 hairs or barbs upon it, it is firmly held in place. The large 

 barbs, which are so conspicuous a feature of the exploded 



Text-fig. 3. 



Discharged nematocyst. 



nematocyst, are never found imbedded in the tissues of the 

 prey. Iwanzoff (6) says that the three, just at the moment of 

 their eversion, form a stiletto which pierces the cuticle of the 

 prey, making a hole by which the thread can enter. With the 

 further eversion of the thread the barbs swing outward to their 

 final position. Some chemical action is exerted on the chitin, 

 as can be seen from a study of sections. Immediately around 

 the point of entry is a deeply staining area, irregular in form 

 (Text-fig. 3, 1). This probably corresponds to an outpouring 

 of the poisonous fluid contained in the nematocyst, for the 

 thread stains in the same way. Outside this dark patch is 



