248 COELENTERATA 



rejected from the body, and its place in the tissue is taken by a 

 new nematocyst formed by a new cnidoblast ; but in the thread 

 of the large kind of nematocyst of Millepora there is a very 

 delicate band, which appears to be similar to the myophan 

 thread in the stalk of a Vorticella. Dr. Willey ^ has made the 

 important observation that in this coral the nematocyst threads 

 can be withdrawn after discharge, the retraction being effected 

 with great rapidity. The " cnidoblast " is a specially modified 

 cell. It sometimes bears at its free extremity a delicate process, 

 the " cnidocil," which is supposed to be adapted to the reception 

 of the special stimuli that determine the discharge of the nema- 

 tocyst. In many species delicate contractile fibres (Fig. 124, 

 Mf) can be seen in the substance of the cnidoblast, and in others 

 its basal part is drawn out into a long and probably contractile 

 stalk ("cnidopod"), attached to the mesogloea below. 



There can be little doubt that new nematocysts are constantly 

 formed during life to replace those that have been discharged 

 and lost. Each nematocyst is developed within the cell- sub- 

 stance of a cnidoblast which is derived from the undifferentiated 

 interstitial cell -groups. During this process the cnidoblast 

 does not necessarily remain stationary, but may wander some 

 considerable distance from its place of origin.- This habit of 

 migration of the cnidoblast renders it difficult to determine 

 whether the ectoderm alone, or both ectoderm and endoderm, 

 can give rise to nematocysts. In the majority of Coelenterates 

 the nematocysts are confined to the ectoderm, but in many 

 Anthozoa, Scyphozoa, and Siphonophora they are found in tissues 

 that are certainly or probably endodermic in origin. It has not 

 been definitely proved in any case that the cnidoblast cells that 

 form these nematocysts have originally been formed in the 

 endoderm, and it is possible that they are always derived from 

 ectoderm cells which migrate into the endoderm. 



It is probably true that all Coelenterata have nematocysts, 

 and that, in the few cases in which it has been stated that they 

 are absent (e.g. Sarcophytum), they have been overlooked. It 

 cannot, however, be definitely stated that similar structures do 

 not occur in other animals. The nematocysts of the Mollusc 

 Aeolis are not the product of its own tissues, l)ut are introduced 



1 JVilhifs Zool. Results, pt. ii. 1899, p. 127. 

 - Murbach, Archiv f. Xatunj. l.x. Bd. i. 1894, p. 217. 



