iv PHYLUM CCELENTERATA 97 



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 

 protoplasmic 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 

 undergoes 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 Medusae 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. 40, 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 (tnnb.\ at the free end of which is 

 the four-sided mouth (wth.}. Very commonly as the 

 medusa swims the umbrella becomes turned inside out, the 

 sub-umbrella then forming the convex surface, and the 

 manubrium springing from its apex (Fig. 40, C). 



The mouth (Figs. 40, C, Z>, and 42, mth.) leads into an 

 enteric cavity which occupies the whole interior of the 

 manubrium, and from its dilated base sends off four delicate 

 tubes the radial canals (rad. c.\ which pass at equal 

 distances from each other through the substance of the 

 umbrella to its margin, where they all open into a circular 

 canal (circ. c.}, running parallel with and close to the 



H 



