28 



ANIMAL HJOLOGY 



V 



Fig. 40.— Hydroid Colony, with 

 nutritive (P) reproductive (^l/)and 

 defensive (S) hydranths. 



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 

 jelly. This shows sensibility, 

 and a few small star-shaped 

 cells 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 Coelen- 



tcrata). — 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 hydroids, 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. 



