iv PHYLUM CCELENTERATA 95 



no cell-structure, but only a delicate lamination. It is, in 

 fact, not a cellular membrane or epithelium, like the ecto- 

 derm and endoderm, but a cuticle, formed, layer by layer, 

 as a secretion from the ectoderm cells (see p. 67). It is 

 of chitinoid or horn-like consistency, and, like the lorica 

 of many Protozoa, serves as a protective external skeleton. 



Embedded in the ectoderm are numerous clear, ovoid 

 bodies, the stinging-capsules or nematocysts (Fig. 42), serving 

 as weapons of offence. Each consists (A) of a tough, ovoid 

 capsule, full of a gelatinous material, 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 ap- 

 proaches maturity, migrates towards the surface, and becomes 

 embedded in one of the large ectoderm cells. At one point 

 of its surface the cnidoblast is produced into a delicate pro- 

 toplasmic process, the cnidocil or trigger-hair (cnc) : when this 

 is touched — for instance by some small organism brought 

 into contact with the waving tentacles — the cnidoblast un- 

 dergoes a sudden contraction, and the pressure upon the 

 stinging-capsule causes an instantaneous eversion of the 

 thread (B), at the base of which are minute barbs. The 

 threads or the gelatinous substance are poisonous and 

 exert a numbing effect on the animals upon which the 

 Obelia preys. 



The structure of the Medusa? — formed as we have seen 

 by the development of medusa-buds liberated from a 

 ruptured gonangium — yet remains to be considered. The 

 convex surface of the bell or umbrella (Fig. 41, B-D) by 

 which the zooid was originally attached to the blastostyle, is 

 distinguished as the ex-umbrella, the concave inner surface 

 as the sub-umbrella. From the centre of the sub-umbrella 

 proceeds the manubrium (mnb), at the free end of which is 



