CLASS HYDROZOA. 1 3 



and extensible processes, which are termd the "tenta- 

 cles " (Latin, tento, I touch). By means of these feelers 

 the animal seizes its prey. 



The mouth opens into a long cylindrical cavity, which 

 occupies the whole length of the body, and which receives 

 the food. This cavity may therefore be regarded as the 

 stomach; though it is reaUy the general cavity of the 

 body, since it is enclosed by the general integument. 

 Nothing further can or need be said about the internal 

 anatomy of the Hydra; since the undigested food is 

 simply expelled through the mouth, and there are no traces 

 of a nervous system, of breathing-organs, or of any circu- 

 latory apparatus. 



The walls of the body in the Hydra are very soft and 

 contractile, and the animal can pull itself together into a 

 shapeless lump when irritated, or can expand 

 itself and thrust out its tentacles when in 

 search of food. The outer layer of the body, 

 and especially of the tentacles, is roughened 

 by innumerable little microscopic bodies, 

 which are known as "thread -cells" or 

 "nettle -cells." These ciuious little organs 

 (fig. 4) are seen, when highly magnified, to 

 consist of a little bladder filled with fluid, 

 and carrying at one end a long filament or 

 thread. This thread can be darted out with 

 great rapidity and force, and it is used by 

 the animal in capturing its prey. The 

 thread-cells are too weak to pierce the human ^\^{~^l^^y. 

 skin, but they wound the soft bodies of the rfm; much 

 worms and other minute animals upon which ™asii'fle<^- 

 the Hydra feeds, and they appear also to exercise some 

 poisonous or benumbing effect. 



In the summer time, the Hydra produces young ones 

 by a process of budding, as is seen in fig. 3, A, much as a 

 plant throws out buds ; but these young are detached to 

 lead a separate life, when they are fully grown. In the 

 autumn the Hydra also produces eggs. 



The Common Hydra is found abundantly in most 



