PRACTICAL ZOOLOGY 33 



Note the effect of light on hydras by putting several 

 in a jar of water and covering it with an opaque paper 

 through which on one side a hole one inch in diameter 

 has been made, the jar then being placed near a win- 

 dow and the hole being directed toward the light. 



For comparison use the sea anemone, Metridium 

 (Fig. 236). , 



Campanularian Hydroid 



Material. Specimens of Eucope or Obelia are found 

 attached to seaweed or submerged timbers, below low 

 tide mark, in the sea. The colonies are usually grayish 

 in color, much branched, and have a noticeably plant- 

 like aspect. If living specimens are available for 

 study, they may be placed in small dishes of fresh 

 sea water and examined with a magnifying glass. 

 The various motions of the polyps may be studied, 

 their protrusion from and withdrawal into the pro- 

 tective cup which surrounds each one, the rapid ex- 

 tension and twisting of the tentacles, the protrusion 

 of the funnel-like mouth to ingulf particles of food 

 (minute scraps of fresh meat are suitable), the sensi- 

 tiveness of the various members of the colony to 

 jarring, touching with a bristle point, agitation of the 

 water, and so on (Fig. 20). 



It will be noticed that the colony consists of two 

 forms of zooid, one bearing tentacles, the nutritive 

 zooids ; the other being without tentacles, elongated 

 in shape, and containing a number of rounded bodies. 

 This form is the reproductive zooid, and the contained 

 bodies are the medusa buds or medusoids, which, when 

 mature, are liberated and produce the eggs from which 

 new, branched colonies arise. The phenomenon of 

 "alternation of generations" is here very marked. 



The microscopic structure is best seen in mounted 

 specimens, which may be obtained from dealers. 

 DODGE'S GEN. ZOOL. 3 



