IV 



PHYLUM CCELENTERATA 



123 



cells (Fig, 86, nv.c). The tentacles and the manubrium contain, 

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

 and the mesogloea : they are arranged longitudinally, and serve 

 for the rapid shortening of the tentacles (Fig. 86, m.f). This 

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

 upon as a rudimentary mesoderm. 



nb 



FIG. So. Xematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb. 

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

 Biolorry, after Schneider.) 



Embedded in the ectoderm are numerous clear ovoid bodies, the 

 stinging-capsules or nematocysts (Figs. 83 86, ntc), organs closely 

 resembling those of Epistylis umbellaria (p. 84), and like them 

 serving as weapons of offence. Each consists (Fig. 85, 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 in an interstitial cell called a 

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



