HYDROZOA 19 



The food is obtained by the viscid tentacles, which, when the 

 Hydra is undisturbed, are extended (as is usually the body), 

 ready to grasp the prey, for this tiny animal is carnivorous, 

 feeding upon small organisms, usually crustaceans. There are 

 nettle cells, or nematocysts, in the ectoderm of the tentacles. 

 When an animal comes in contact with a tentacle, the nemato- 

 cysts near the point touched throw out stinging threads which 

 partially paralyze the animal by the fluid which they discharge 

 into the wound they have pierced. The tentacles then pass 

 the prey to the mouth, which opens into the gastrovascular 

 cavity, in which digestion is carried on and into which the 

 wastes are gathered and thrown out through the only opening, 

 the mouth. The Hydra, by its wide-open mouth and envelop- 

 ing lips, often takes in organisms much larger than itself. 



Nerve-cells, sex-cells, and nettle-cells are situated in the 

 ectoderm. 



Multiplication in the Hydra is both sexual and asexual. 

 It reproduces by budding, but as the buds mature they become 

 detached, so that no permanent colony is formed. It also 

 reproduces by eggs, the animal being hermaphroditic, that is, 

 the reproductive organs of both sexes are found in the same indi- 

 vidual. Near the base of the tentacles are found the spermaries 

 from which the sperm cells are discharged into the water; the 

 ovaries are situated farther down, near the lower end of the 

 body. The eggs are cross-fertilized, that is, fertilized by the 

 sperm cells of another individual. After fertilization the ova 

 remain in the ectoderm for some time, when they become en- 

 cysted in spiny cysts, drop off into the water, and sink to the 

 bottom. They lie here till the following spring, when they 

 break their casing and come forth as minute Hydras. In the 

 encysted condition they are able to withstand cold and drouth, 

 thus insuring the perpetuation of the species. Hydras also have 

 the power of regenerating the whole body from a part in case of 

 injury. 



Locomotion. — The Hydra is temporarily fixed by adhering 

 to the submerged stems of water plants by means of a sticky 

 secretion from the closed end of the tube. It can detach itself, 

 and, by grasping with its tentacles, can pull itself up and again 

 attach the end of its tubular body to an object. By this cater- 



