412 



EVENINGS AT THE MICROSCOPE 



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 ab- 

 normal 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 at- 

 tained 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 Sa- 

 gartia parasitica. 



"When fully expelled, the thread or 

 wire, which is distinguished by the term 

 ecthorceum, is often twenty, thirty, or even 

 forty times the length of the cnida ; though 

 in some species, as in most of the Sagartice, 

 I * it frequently will not exceed one and a 

 / \ half, or two times the length of the cnida. 



CNIDA OF T . CRASSI- " T}iG ecthor^a, which o,TG discharged by 

 (discharged). c ^ am j em j cnidce, are invariably furnished 

 with a peculiar armature. The basal portion, for a length 

 equal to that of the cnida, or a little more, is distinctly 

 swollen, but at the point indicated it becomes (often 



