NEMATOCYSTS 



247 



This is an organ produced within the body of a cell called 

 the " cnidoblast," and it consists of a vesicular wall or capsule, 

 surrounding a cavity filled with fluid containing a long and 

 usually spirally coiled thread continuous with the wall of the 

 vesicle. When the nematocyst is fully developed and receives 

 a stimulus of a certain character, the thread is shot out with 

 great velocity and causes a sting on any part of an animal that 

 is sufficiently delicate to be wounded by it. 



The morphology and physiology of the nematocysts are 

 subjects of very great difficulty and complication, and cannot be 

 discussed in these pages. It may, however, be said that by 

 some authorities the cnidoblast is supposed to be an extremely 

 modified form of mucous or gland cell, and that the discharge of 

 the nematocyst is subject to the control of a primitive nervous 

 system that is continuous through the body of the zooid. 



There is a considerable range of structure in the nemato- 

 cysts of the Coelenterata. In Alcyonium and in many other 

 Alcyonaria they are very small (in Alcy- 

 onium the nematocyst is 0*0075 mm. in 

 length previous to discharge), and when 

 discharged exhibit a simple oval capsule 

 with a plain thread attached to it. In 

 Hydra (Fig. 124) there are at least two 

 kinds of nematocysts, and in the larger 

 kind (0-02 mm. in length previous to dis- 

 charge) the base of the thread is beset 

 with a series of recurved hooks, which 

 during the act of discharge probably assist 

 in making a wound in the organism 

 attacked for the injection of the irritant 

 fluid, and possibly hold the structure in Fig. 124 

 position while the thread is being dis- 

 charged. In the large kind of nematocyst 

 of Millepora and of Cerianthus there is a 

 band of spirally arranged but very minute 

 thorns in the middle of the thread, but 

 none at the base. In some of the Siphono- 



phora the undischarged nematocysts reach their maximum size, 

 nearly 0*05 mm. in length. 



When a nematocyst has once been discharged it is usually 



Nematocyst 

 (Nem) of Hydra grisea, 



enclosed within the cnido- 

 blast. GNG, Cnidocil ; 

 /, thread of nematocyst ; 

 Mf, myophan threads in 

 cnidoblast ; N, nucleus 

 of cnidoblast. (After 

 Schneider.) 



