TYPES OF CCELENTERA HYDRA. 143 



threads is mechanical and chemical. They fix, e.g. by the stilets, into 

 the victim, and the secretion poisons the wound, paralysing or killing 

 small animals, and sometimes acting as a solvent. Many seem to be 

 prehensile threads rather than weapons. 



TYPES OF CCELENTERA 



First Type HYDRA, a simple representative of the 

 Class HYDROZOA 



General life. Thegenus/^foz cosmopolitan, like many 

 other small fresh-water animals is represented by several 

 species, e.g. the green Hydra viridis^ the brownish H. oligactis 

 or fusca, and the orange H. vulgar is 

 or grisea, widely distributed in fresh 

 water. They are among the simplest 

 of Coelentera, 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 reproduction. 

 The body is usually fixed by its base 

 t" some aquatic plant, often to the 

 lower surface of a duckweed. 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 up by its tentacles, stand on 

 its head, or creep by looping movements. According 

 to some observers, its movements are helped by fine 

 pointed pseudopodia protruded from the ectoderm cells 

 of the tentacles and base, and by threads ejected from 

 large cylindrical stinging cells. Usually, however, the Hydra 

 prefers a quiet life. It feeds on small animals, 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 the 

 Hydra may sometimes be seen struggling fiercely with 

 a small Annelid worm (Tubifex). Infusorians (Euplotes^ 



FIG. 68. Hydra hang- 

 ing from water-weed. 

 After Greene. 



0v. t Ovary; /., testes. 



