IV 



PHYLUM CCELENTERATA 



133 



edU (Fig. 98, nv.c). The tentacles and the manubrium contain, 

 in addition, a layer of unstriped muscle-fibres between the ectoderm 

 and the mesoglcea : they are arranged longitudinally, and serve 

 for the rapid shortening of the tentacles (Fig. 98, inf). This 

 muscular layer is a derivative of the ectoderm, and may be looked 

 upon as a rudimentary mesoderm. 



Fig. 97.— Nematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; otft, 

 cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nematocyst ; nv.c. nerve-cell. (From Parker's 

 Biology, after Schneider.) 





Embedded in the ectoderm are numerous clear ovoid bodies, the 

 stinging-capsules or nematocysts (Figs. 96 — 98 ntc), organs closely 

 resembling those of Epistylis umbellaria (p. 93), and like them, 

 serving as weapons of offence. Each consists (Fig. 97, A) of a tough 

 ovoid capsule, full of fluid, and invaginated at one end in the form 

 of a hollow process continued into a long, coiled, hollow thread. 

 The whole apparatus is developed into an interstitial cell called a 

 cnidoblast (cnb), which, as it approaches maturity, migrates towards 



