BRITISH ZOOPHYTES. 23 



hydra is exceedingly mutable, the animal having the 

 power of changing its shape iu the most protoean 

 manner. At one time it appears fully extended, long 

 and thin ; at another time it is rolled up like a ball. 

 The substance of which the body wall is composed is 

 sarcode. This body wall consists of two layers, the 

 ectoderm (ec) and the endoderm {en). The ectoderm 

 contains two kinds of cells, large conical ones with the 

 broad end outwards, and smaller round cells between 

 the tapering ends of the larger ones. 



Among the cells of the ectoderm are what are 

 variously called urticating capsules, nematocysts, cnidae, 

 or thread cells. These cells are a peculiarity of the 

 Coelenterata. They are oval bags, each containing a 

 thread or filament capable of being suddenly extended 

 on the slightest pressure. Some of the cells contain a 

 sheath provided with three recurved bai"bs, round 

 which the thread is coiled : the barbs probably act as 

 a spring. These cells appear to secrete a poisonous 

 fluid, which, from the fact that it deposits metallic 

 silver from the nitrate of that metal, is believed to be 

 formic acid. The threads on extrusion pierce the body 

 of the object which the hydra wishes to secure as its 

 prey, and convey into it the poison, to the effect of 

 which it speedily succumbs. The threads assume 

 varied forms in the different species of Ccelenterata j 

 and even in the same species there is often great 

 diversity. In Plate I. figs. 3, 4, 5, are shown diagrams 

 of the thread cells with the threads at rest and exserted. 

 Between the ectoderm and the endoderm is a layer of 

 fibres which are prolongations of the larger ceils of 

 the ectoderm, and appear to serve the purpose of 

 muscles. Between the muscular layer and the endo- 



