388 The Animal Kingdom 



than those of the epidermis. Within both layers may be found special 

 epitheliomuscular cells, sensory cells, interstitial cells, and gland cells. 

 The epitheliomuscular cells are the most abundant and represent a regu- 

 lar, though very simple, muscular system. The outer fibers are longitu- 

 dinal and are located at the base of the epidermis ; the inner ones are 

 circular and are at the base of the gastrodermis. The longitudinal fibers 

 serve to contract either the whole animal or parts of it, while the circular 

 ones tend to constrict and thus would serve to aid the extension of the 

 body. 



Several tall gland cells are found in the basal disc where they func- 

 tion for attachment by secreting mucus. These also can secrete gas 

 bubbles which allow the animal to float. 



For the reception of stimuli within its environment and the coor- 

 dination of the activities of the body and tentacles, the hydra has a net- 

 work of nerve cells which are located in the epidermis and the mesoglea, 

 with a few being found in the gastrodermis. Scattered throughout the 

 epidermis are the small sensory cells. These are most abundant about 

 the mouth, the basal disc, and on the tentacles. As was noted above, 

 they are connected directly to the nerve cells at their bases. Some of 

 these sensory cells have flagellar processes. 



Located among the epidermal cells are small undifferentiated cells, 

 the interstitial cells. These are capable of giving rise to other types of 

 cells such as buds, sex cells, and the stinging cells or nematocysts. 

 These latter are scattered over the body except on the basal disc, but 

 are most abundant on the tentacles where they occur in batteries. 



Reproduction. — Hydra is one of the members of the class Hydro- 

 zoa in which the jellyfish or medusoid stage is completely absent. Thus 

 there is no true alternation of generations. New individuals are pro- 

 duced in one of two ways, either by budding or by sexual reproduction. 



Buds are usually produced just above the basal disc and are first 

 evidenced as thickenings of both body layers. A rounded protuberance 

 appears, soon elongates, and tentacles appear at the free end. Nourish- 

 ment is provided for by the gastrovascular cavity of the parent which 

 is continuous with that of the bud. Development of the bud is com- 

 pleted before it frees itself from the parent. Under favorable conditions, 

 a single hydra may have several buds with the oldest at the base. Bud- 

 ding occurs at any season, and is partially dependent upon favorable 

 weather and available food. 



