134 



CCELENTERA. 



cilia united into combs, and often shine with that " phos- 

 phorescence " which is an expression of intensity of life in 

 many active animals. The Ctenophores have probably 

 arisen from a modified Hydrozoon medusoid. 



As to diet, the active Ctenophores are carnivorous, 

 attaching themselves by adhesive cells to one another, or to 

 other small animals ; many of the larger forms, e.g., sea 

 anemones and jelly fish are able to engulf booty of con- 

 siderable size ; the majority, however, feed on small organ- 

 isms, in seizing and killing which the tentacles and stinging 

 cells are actively used, but what the^corals eat no one seems 

 to know. 



TYPES OF CCELENTERA. 

 First Type HYDRA, illustrative of the Class HYDROZOA. 



General Life. The genus Hydra is represented by 

 several species, e.g., the green Hydra vindis and the 

 brownish Hydra fusca, both widely 

 distributed in fresh water. It is one 

 of the simplest of Ccelentera, for the 

 body is but a two-layered tube, with 

 a crown of (6-10) hollow tentacles 

 around the mouth, and with no organs 

 except those concerned in reproduc- 

 tion. The body is usually fixed by 

 its base to some aquatic plant, often to 

 the underside of a duckweed leaf. It 

 may measure \-\ inch in length, but it 

 is as thin as a needle, and contracts 

 into a minute knob. 



The animal sways its body and 

 tentacles in the water, and it can also 

 loosen its base, lift itself by its ten- 

 tacles, stand on its head, or creep by 

 looping movements. Usually, how- 

 ever, it prefers a quiet life. It feeds on small organisms, 

 which are paralysed or killed by stinging cells on the 

 tentacles, and are swept into the tubular cavity of the body 

 by the action of flagella on the internal cells. Sometimes 

 animals as large as water fleas (e.g., Daphnid) are caught, 

 and in part digested. Infusorians (Euplotes, etc.) are often 



FIG. 39. Hydra, 

 hanging from water 

 weed. 



(After GREENE.) 



ov, Ovary. /, Testes. 



