420 



EVENINGS AT THE MICEOSCOPE. 



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 ob- 

 servation is pretty sure to present some fortuitous 

 combinations, and abnormal conditions, which gi'eatly 

 elucidate phenomena, that normally seemed to defy 

 investigation. 



" In watching any particular cnida, 

 the moment of its emission may be pre- 

 dicted 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 dis- 

 tinct crack or crepitation, both in the 

 examination of this species and of Sa- 

 gartia parasitica. 



"When fully expelled, the thread 

 or wire, Avhich is distinguished by the 

 term ecthorcBum, 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 

 CNIDA OP B. cEAssi- the IcHgth of tho cnida. 



C0EN18, discharged. y' 



ihe ecthorcBa which are discharged 

 by chambered cnidm, are invariably furnished with a 

 peculiar armature. The basal portion, for a length 



