656 



Comparative Animal Physiology 



like process by eversion (Fig. 248). There is usually a cap or operculum 

 covering that region of the undischarged nematocyst through which the 

 thread is ejected. The nematocysts are produced within interstitial cells either 

 at their final site or at some distance from this site, to which they move by 

 ameboid activity or passive transportation. A cell containing a nematocyst 

 is known as a cnidoblast or nematocyte. The portion of the cnidoblast con- 

 taining the nematocyst eventually comes to occupy a superficial position and 

 usually develops at its outer end a bristle-like projection, the cnidocil. The 

 latter is imbedded in a small crater-like elevation on the cell. The cnidoblast 

 also often difi^erentiates trichite-like supporting rods in its peripheral regions 

 and often, too, a fibrillar network associated with the capsule of the nemato- 

 cvst and extending proximally from it. 



The nematocvsts of coelenterates are divisible into two major groups, the 

 spirocysts of the Zoantharia, which are acid-staining bodies possessing peculiar 

 adhesive threads, and the basic-staining nematocysts proper. The latter are 

 widely distributed through the whole phylum. They are of many types in the 

 characters of their discharged threads. In some, e.g., the volvents, the thread 

 is closed at its tip and forms a tightly coiled filament on discharge, wrapping 



Fig. 248. An undischarged (a) and a discharged (b) nematocyst located in its cnidoblast. 

 The spine-like projection of the cnidoblast is the cnidocil. Redrawn from Sedgwick. 



itself about bristles or fibers of organisms in whose presence they are dis- 

 charged. Most other types have open tips and are believed to penetrate prey, 

 injecting into them a toxic substance. The threads of these latter .types com- 

 monly possess an armature in the form of three spiralling rows of spines 

 which serve effectively to anchor the thread into tissues which have been 

 penetrated. The penetrating capacity of at least some types is so great that 

 even the chitinous cuticles of small organisms can be punctured. 



The sting of numerous species gives rise to severe itching and other skin 

 disorders in man. The toxicity is very great in a few coelenterates such as 

 Physalia, and some larger scyphozoan jellyfish such as Dactylometra, even 

 enough to render them highly dangerous to man. Their sting may in some 



