THE PHYLA COELENTERATA AND CTENOPHORA 



185 



A B 



Figure 10.4. A, A portion of a tentacle from Gonionemiis, showing rings of nemato- 

 cysts. (After Hyman.) B, Gonionemus veilens actively swimming. (Courtesy Douglas P. 

 Wilson.) 



A. Undischarged B. Discharged 



Figure 10.5. Diagrammatic view of a nematocyst. A, Before discharge. B, Right, 

 Everted. 



the thread appears in an inverted position, like a glove finger pulled 

 inside out. To accommodate its length the thread is tightly coiled. On 

 the outer surface of each nematocyst is a tiny projecting trigger. Upon 

 suitable stimulation, which appears to include taste in addition to a 

 touch on the trigger, the nematocyst fires. 



Firing is explosive. The nematocyst absorbs water, which increases 

 the internal pressure and everts the thread, just as a pulled-in glove 

 finger can be everted by blowing into the glove. The diameter of the 

 thread is so small and its eversion so fast that it easily penetrates the tis- 

 sue of the prey. After discharge (Fig. 10.5) the everted thread is seen to 

 bear recurved hooks on a swollen base and to be open at the tip. The 

 hooks hold the prey fast while the poisonous contents of the nematocyst 

 are discharged through the thread into its body. 



Gonioyiemiis has just one kind of nematocyst. Other coelenterates 

 have several distinct kinds, with marked differences in the details of 

 hooks and thread. 



/ngesf/on. Having caught its prey the medusa shortens its tentacles 



