3Q6 EVENINGS AT THE MICROSCOPE. 



resides in a very high degree in the interior of the cnidcp : 

 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 

 cnida. But sometimes impediments delay the emission, 

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

 ])ortion' ; 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, 

 becomes 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 

 abnormal 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 

 extremity. 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 Sagartice, it 

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

 the length of the cnida. 



The ecthorea which are discharged by chambered cnida 3 



