90 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



the surface of the tentacle into the surrounding water. The most remark- 

 able feature of the cnidoblast is the nematocyst {new) which fills up a 

 great part of its interior. This is a hollow flask or bulb the neck of which 

 tapers off into an extremely fine tube open at its free end (Fig. 35, C). 

 Round the neck of the bulb are arranged three sharp blades. 

 In the nematocyst as observed within the cnidoblast the fine 

 tube, including the neck with its blades, is turned outside in 

 and lies in the interior of the bulb, the tube being coiled 

 up into a spiral. The rest of the cavity of the bulb is filled 

 with fluid, apparently of a virulently poisonous kind. 



Besides the cnidoblasts such as that which has been 

 described there exist others, more numerous, smaller in size 



Tiem 



ABC 



Fig. 35. 



Cnidoblast and nematocysts. A, Unextnided ; B, early stage of extrusion ; C, extrusion 

 complete, en, Cnidocil ; «, nucleus ; nem, nematocyst. 



and containing nematocysts of slightly different shape and unprovided 

 with blades. 



The cnidoblasts are originally interstitial cells which gradually form 

 the nematocyst in their interior. They then creep away from their 

 point of origin, usually though not always into the tentacle, burrow their 

 way into the substance of an ordinary ectoderm cell and there settle 

 down, pushing their cnidocil beyond the surface of the host cell into 

 the surrounding water. In a normal ectoderm cell of the tentacle 

 there is usually a group or battery of cnidoblasts, a larger one in the 

 centre and a circle of smaller ones round it. 



