198 



THE ANIMAL KINGDOM 



to break the brittle skeleton and gather the branches. Other alcyonarians 

 are the yellow, red or purple sea fans (Fig. 10.16) of tropical waters. 

 These colonies develop as llattened networks with a few main branches 

 and numerous cross connections. 



69. Fresh-Water Coelenterates: Hydra 



Only a few species of coelenterates, all members of the class Hy- 

 drozoa, occur in fresh water. The fresh-water forms include a colonial 

 polyp found in a few eastern rivers of the United States, a jellyfish very 

 similar to Gonionemus found sporadically in ponds and streams all over 

 the world, and a number of species of solitary polyps, the hydras. 

 Only the last are easily obtained in most bodies of fresh water. 



The hydra {¥ig. 10.17, left) is an unusual hydrozoan. Medusae are 

 lacking altogether, unless the gonads are considered to be their vestiges. 

 As in Gonionemus the sexes are separate (Fig. 10.18). The testes shed 

 sperm into the water and each ovary produces one egg at a time which 

 is retained and fertilized in the ovary. The egg develops to the planula 



Figure 10.17. Left, Hydra. The tentacles hang in the water like a net, waiting for 

 prey. (After Hyman.) Right, Spine of a crustacean that has brushed against the tentacles 

 of a hydra. Two of the nematocysts shown are similar to those of Gonionemus. The 

 others are of the coiling type. (After Hyman.) 



