144 TEXTBOOK OF ZOOLOGY 



takes a bilateral form, and the individuals are born on a disc or axial 

 stem which is supported by a hard skeleton. There may be dimor- 

 phism of zooids within the colony. Renilla and Pennatula, sea pens 

 and sea feathers, are typical examples. 



HYDRA 



Habitat and Behavior 



Hydra (Chlorohydra) viridissima is likely the most common hydra 

 of the Southwest. It is the small green hydra which is very 

 active and has short tentacles. This species has the green color 

 because of the presence of a unicellular alga, Chlorella vulgaris, in 

 the endoderm cells. The plant uses some of the by-products of 

 metabolism of the hydra, and the hydra benefits by receiving oxygen 

 from the photosynthesis of the alga. This kind of a relationship is 

 called symbiosis. 



Most of the hj^dras are found in cool fresh water, attached to the 

 surface of plant leaves, smooth sticks, debris, or even the surface film 

 of the water. The brown hydras, such as H. americana, H. carnea, 

 and Pelmatohydra oligactis, are sluggish and have longer tentacles 

 than the green ones.* 



Hydra is a sedentary kind of animal and may remain stationary 

 for a considerable period of time if living conditions are uniformly 

 good. When the environmental conditions are changing, and the 

 animal is in need of food, it becomes quite active, moving about 

 from place to place. It keeps the tentacles extended, ready to 

 grasp any food which may come into its reach. Nematocysts or 

 sting bodies are discharged when the tentacle comes in contact 

 with potential food, and if it chances to be a small animal, it will 

 likely be paralyzed by the toxin which is injected by the nemato- 

 cysts. The prey is then carried to the mouth and tucked into it by 

 the tentacles. Frequently hydra is able to stretch its body over 

 articles of food which are actually larger than the hydra usually 

 is in normal condition. Hydra will eat only when it is hungry and 

 will not react to food at other times. It is more sensible than many 

 people in this respect. On the other hand, it has been authentically 

 reported that a hungry hydra will perform the characteristic feed- 



♦Recent taxonomic information concerning Hydras of the United States may be 

 found in the papers of Libbie H. Hyman, published in tlie Transactions of the 

 American Microscopical Society, Vols. 48, 49, and 50. 



