28 



ANIMAL BIOLOGY 



body at each step. Sometimes 

 the body goes end over end in 

 slow somersaults. 



The length of the extended 

 hydra may reach one half 

 inch. When touched, both 

 tentacles and body contract 

 until it looks to the unaided 

 eye like a round speck of 



icily. This shows sensibility, 

 FIG. 40. HYDROID COLONY, with J 



nutritive (P) reproductive (M) and and a few Small Star-shaped 



defensive (S) hydranths. ce]ls are believed to be nerve 



cells, but the hydra has not a nervous system. Hydras 

 show their liking for light by moving to the side of 

 the vessel or aquarium whence the light comes. 



The Branch Polyps 

 (sometimes called Ccelen- 

 teratd). The hydra is 

 the only fresh water rep- 

 resentative of this great 

 branch of the animal 

 kingdom. This branch 

 is characterized by its 

 members having only 

 one opening to the body. 

 The polyps also include 

 the salt water animals 

 called hydro ids, jelly- 

 fishes, and coral polyps. 



Hydroids. - - Figure 40 

 shows a hydro id, or 

 group of hydra-like 

 growths, one of which 



FIG. 41. " PORTUGUESE MAN-O'-WAR" 

 (compare with Fig. 40). A floating 

 hydroid colony with long, stinging (and 

 sensory) streamers. Troublesome to 

 bathers in Gulf of Mexico. Notice 

 balloon-like float. 



