350 ELEMENTARY BIOLOGY. [CHAP. 



a. The cnidocils ; well defined stiff processes, each 

 pointed and standing out from the free surface. 



b. The nematocysts or thread-cells; highly refractive 

 bodies embedded in the common mass. They can 

 be resolved into 



a. Smaller and more numerous ones; ovoidal, and 

 situated almost invariably at the bases of the 

 longer cnidocils. 



ft. Larger and less numerous ones (very often one 

 only) lying near the middle of the mass ; globular 

 when seen en face, flask-shaped when looked at 

 from the side. 



c. Run in a drop of acetic acid magenta solution and 

 watch, under a low power, the ejection of the threads 

 of the nematocysts. Note from what parts of the 

 body they are thrown out. Examine, under a high 

 power, a portion of a tentacle beset by them. There 

 will be found : 



a. Short, deeply staining, stout threads, usually thrown 



into a spiral. Note that these are related to the 



smaller nematocysts. 

 ft. Delicate whip-lash filaments, from 8 to 10 times 



the length of a. Trace them to their origin from 



the larger nematocysts. 



d. Should opportunity offer, examine any small or- 

 ganism or other prey that may have been seized 

 by the tentacles. When about to be swallowed it 

 will be found to be studded by nematocysts, espe- 

 cially of the smaller kind. 



4. The asexual buds. (Cf. Sect. 3 a. S) Examine in 

 relation to the parent, under a high power. 



