THE HYDRA AND ITS ALLIES 



207 



the lumen of the thread. If a small animal has been the 

 irritant, it is instantly caught in the thread, paralyzed by 

 the poison, and soon conveyed to the Hydra's mouth. 

 The principal food of Hydra is small worms and the 

 smaller Crustacea, such as Daphnia and Cyclops. 



FIG. 193. A colony of Cordylophora lacustris, on a shell of Mytilus. For 

 clearness, a number of the erect branches are cut off at the stolon, a, very 

 young 1 shoot without lateral branches; b, young stock with lateral branches 

 but no gonophores ; c, a stock with gonophores on lateral branches ; d, fully 

 grown stock with lateral stems. After Schulze. 



Fresh-water Cnidaria are of extremely few kinds. 

 Hydra was once regarded as the only instance, but 

 others are now known. One of these, called Cordylo- 

 phora, is found chiefly in brackish water or in fresh water, 

 near the coast. Like many of the marine hydroids, Cordy- 



