PHYLUM CCELENTERATA 



133 



cells (Fig, 98, nv.c). TViA_fpnt,np1pg anH t.ViP-_majujluiiu.r 



ti Tadd iti^n. a. Ipjftr of unstriprrl mufirJf-fihrfi >>"trW^Ti- the- ectoderm 

 and the_ TTipsngl npni • tViry nro nTmngprl Iffngitiidinflllj,— '^J^"^-"^'^"' 



fnr t.hp rapid sVinr-tiMwig- of -thtr'-teil-taeles -(F-igr-967 Wirj^). This 



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

 upon as a rudimentary mesoderm. 



cf 



Fig. 97. — Nematocysts of Hydra. A, undisch<arged ; B, discharged ; C, nerve-supply ; cnh, 

 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 



