FIELD BOOK OF PONDS AND STREAMS 



join in the team work of paralyzing and pulling in the prey. 

 Hydra swallows its food into a capacious central cavity or 

 ccelenteron, the only cavity in its body. There it is digested 

 and the useless part cast out of the mouth. Hydras are 

 gluttonous feeders and they will eat till their bodies swell out 

 like meal-sacks, the food being clearly visible through their 

 transparent sides (Fig. 89). 



They reproduce both by sex cells and by budding, but the 

 two kinds of reproduction usually occur at different seasons, 



Fig. 89. — Hydra oligactis after a full meal. 



var^'ing with the species and with their living conditions 

 (Fig. 88). Buds are most apt to appear when the animals 

 are well fed. A young hydra bud will stay attached to its 

 parent for some time after it has a mouth of its own and is 

 doing its own eating, but the food which the young one eats 

 may stay in its own coelenteron or it may pass on into the 

 parental food cavity. In sexual reproduction thousands of 

 sperm cells are produced in the spermary, a cone-like eleva- 

 tion near the base of the tentacles (Fig. 88). Only one egg 

 is formed in the ovar\% the larger swelling near the foot, which 

 is less often present than the spermar>\ Spermary and ovary 

 are sometimes present upon the same animal, as they are in 

 the green hydra. Hydra viridissima (Fig. 90), but in most 

 species they are found on different individuals. An egg may 



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