366 EVENINGS AT THE MICROSCOPE. 



sides in a very high degree in the interior of the cnidm ; 

 and the projection of the contents is the result of a vital 

 force. 



In general, the eye can scarcely, or not at all, follow 

 the lightning-like rapidity with which the chamber and its 

 twining thread are shot forth from the larger end of the 

 cnidm. But sometimes impediments delay the emission, 

 or allow it to proceed only in a fitful manner, a minute 

 portion at a time ; and sometimes, from the resistance of 

 friction (as against the glass plate of the compressorium), 

 the elongation of the thread proceeds evenly, but so slowly 

 as to be watched with the utmost ease ; and sometimes the 

 process which has reached a certain point normally, be- 

 comes from some cause arrested, and the contents of the 

 cell remain permanently fixed in a transition state. Thus, 

 a long continued course of patient observation is pretty 

 sure to present some fortuitous combinations, and abnor- 

 mal conditions, which greatly elucidate phenomena, that 

 normally seemed to defy investigation. 



In watching any particular cnida, the moment of its 

 emission may be predicted with tolerable accuracy, by the 

 protrusion of a nipple-shaped wart from the anterior ex- 

 tremity. This is the base of the thread. The process of 

 its protrusion is often slow and gradual, until it has 

 attained a length about equal to twice its own diameter, 

 when it suddenly yields, and the contents of the cnida dart 

 forth. At this instant I have, in many instances, heard a 

 distinct crack or crepitation, both in the examination of 

 this species and of Sagartia parasitica. 



When fully expelled, the thread or wire, which is 

 distinguished by the term ecthoreum, is often twenty, 

 thirty, or even forty times the length of the cnida; 

 though in some species, as in most of the Sagartia, it 

 frequently will not exceed one-and-a-half or two times the 

 length of the cnida. 



The ecthorea which are discharged by chambered cnj,da, 



