126 



escape in a loop or bight, shows that the issue is the result of a 

 merely mechanical action, viz. that of the escaping water. 



The cnidce occur under four distinct forms. 1 . Chambered cnidse 

 (Cnidce earner atce). This is the most widely distributed, and the 

 most elaborately armed. In Cyathina Smithii they occur of com- 

 paratively large size, and are therefore well suited for observation. 

 They are transparent, colourless vesicles, of a long, oval figure, -ai^ tn 

 of an inch in length, and 20 l 00 th in diameter. A fusiform chamber 

 passes through the centre of the anterior moiety, merging at one 

 extremity into the walls of the cnida, and at the other diminishing to 

 a slender chord, which is irregularly coiled within the general cavity. 



Under stimulus the cnidce suddenly expel their contents with 

 great force. In general the eye can scarcely follow the excessive 

 rapidity with which the chamber and its twining thread are shot 

 forth. When fully expelled, the thread, which I distinguish by the 

 term ecthorceum, is often thirty times as long as the cnida ; but in 

 Sagartia generally, it frequently is not more than once and a half 

 the length of the cnida. 



In the ecthorceum from chambered cnidce the basal portion is 

 distinctly swollen ; thence, becoming attenuated, it runs on as an 

 excessively slender wire of equal diameter. Around this basal part 

 wind one or more spiral thickened bands, varying, in different spe- 

 cies, as to their number, the number of volutions made by each, and 

 the angle which the spiral forms with the axis. The direction is 

 from east to north. The spiral armature I call the screw, or strebla. 

 There is no other form of armature than this. 



These thickened spiral bands afford insertion to a series of fine 

 setce, which I call pterygia. These are from eight to twelve in a 

 single volution, and they project in a diagonal direction from the 

 ecthorceum, but often become reverted. In some cases, perhaps in 

 all, the strebla and the pterygia are continued beyond the swollen 

 portion of the ecthorceum, even to the end of the attenuated part. 



2. Tangled cnidse (Cnidce glomiferce). This sort differs from the 

 preceding chiefly in the uniform slenderness of the ecthorceum, which 

 lies coiled up more or less regularly in the cnida, without any cham- 

 ber. Corynactis viridis affords excellent examples for observation. 



3. Spiral cnidse (C. cochleatce). The walls of the tentacles, in a 

 few species, contain very elongated fusiform cnidce, which seem 



