Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 



469 



Fig. 23.2. Hydras in natural positions on water plants and buoyed up beneath 

 the surface film. They swing and stretch downward like pieces of elastic thread 

 frayed out at their ends. 



ceans. They are avid feeders commonly swallowing fingernail clams and eject- 

 ing the shells after the soft bodies have been digested. Attached to the side of 

 an aquarium they hang outward slowly swaying their bodies through the water 

 with their tentacles trailing. Let a water flea graze one tentacle and it instantly 

 shortens, carrying the water flea toward the hydra's mouth while the other 

 tentacles join in paralyzing the victim. The body soon bulges with the water 

 flea whose movements grow feebler as the digestive enzymes begin to work on 

 it (Figs. 23.3, 23.4). 



Common Species. Of the eight species of hydra known in North America, 

 three are widely distributed and common. The green hydra, Chlorohydra 

 viridissima, owes its brilliant color to the single-celled algae called zoochlo- 

 rellae which live within the inner cells of its body. They are thus protected 

 and, during photosynthesis, they use the carbon dioxide that they and the 

 hydra give off in respiration (Fig. 23.4). Two other species are the gray hydra. 

 Hydra americana, in the eastern United States, with short tentacles and no 

 stalk to its body, and the brown hydra, Pelmatohydra oUgactis {Hydra fusca), 

 with a basal stalk and tentacles which stretch three or four times the length of 

 body and stalk combined. Pale-colored hydras are larger, more translucent 

 and better for study than the green ones. 



Fresh-water jellyfishes or medusae (Craspedacusta) have bells about half 

 an inch in diameter. They are rare yet occasionally occur in large numbers as 

 they did in Gardiner's Lake, Connecticut, in the summer of 1952. 



The following account of hydra applies to most of the species. 



