COELENTERATA AND CTENOPHORA 



attachment of the animal is effected. Interspersed between the large epi- 

 dermal cells of the body and tentacles are numerous small interstitial cells 

 and cells called cnidoblasts ("thread formers"), derived from interstitial cells. 

 The sensory cells and nerve cells of the epidermis will be described in the 

 discussion of responsiveness in the hydra. 



The cnidoblasts, containing the nematocysts with which the hydra subdues 

 its prey, are scattered throughout the epidermis but are most abundant upon 

 the tentacles. Nematocysts are one of the most remarkable mechanisms in the 

 Animal Kingdom. They are not cells, but non-living cell products, which 

 might be likened to harpoons, ready to be shot from guns and capable of 

 paralyzing any minute animal which they may strike. The presence of nema- 

 tocysts is one of the distinguishing characteristics of all coelenterates. The 

 bodies of these animals are soft and defenseless, except as some may with- 

 draw within a protective skeleton; but the members of this phylum must be 

 recognized as powerful enemies of the many small animals upon which they 

 prey. The nematocysts of most coelenterates are not harmful to man, although 

 those of some species may be irritating. The poison of a few species can 

 induce a violent reaction of the human skin, serious general symptoms, and 

 even death in sensitive individuals. 



In the development of a nematocyst, an interstitial cell produces in its 

 cytoplasm a minute structure resembling a vacuole; the early stages are so 

 minute that the details of development are not well understood. However, 

 the vacuole is later seen to be a capsule containing a fluid and a thread. The 

 commonly accepted theory is that the thread, hollow in its final form, arises 

 as an ingrowth from one end of the capsule. Clearly, the nematocyst is not 

 a cell but a capsule containing an inverted thread, which is produced and 

 retained until discharge within the cytoplasm of its cnidoblast, a modified 

 interstitial cell. The discharge of a mature nematocyst involves the forceful 

 eversion of the thread, like the turning right-side-out of an inturned glove 

 finger. The sudden entrance of water into the capsule, possibly as a result 

 of some rapid change in osmotic relations or in the permeability of a limiting 

 membrane, is the most plausible explanation of this reaction. The greatly 

 increased internal pressure thus forces the thread out. 



Cnidoblasts may undergo differentiation from interstitial cells in parts of the 

 body remote from the tentacles, in which nematocysts are most commonly 

 used. In this case the cnidoblast is transferred over a considerable distance, 

 coming to rest eventuallv in the epidermis of the tentacle. Here it becomes 

 oriented with its trigger-like projection, the cnidocil, protruding from the 

 general surface of the epithelium. The cnidocil is apparently the part of the 

 cnidoblast most sensitive to external stimulation. 



Because of the absence of nervous connections, the cnidoblasts are con- 

 sidered to be independent effectors, structures responding directly to stimuli 

 and lacking nervous control. Nematocysts are little affected by purely me- 

 chanical stimuli, such as those produced by rubbing the tentacle with a clean 

 glass rod or with a bit of clean blotting paper. Contact with a saliva-coated 



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