Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 477 



a large food-filled cell. It is soft and irregularly shaped, with outspreading 

 processes which are withdrawn as the egg matures. Sperm cells swimming free 

 in the water go through the thin cellular sac enclosing the egg and fertilization 

 occurs while the egg is still- attached to the parent. The now one-celled embryo 

 divides many times and becomes a hollow sphere of cells (blastula), then a 

 double layered sac (gastrula), in the meantime slipping out of its protective 

 sac. The embryo secretes a capsule in which the embryo may remain dormant 

 for several months. There is no evidence, however, that the eggs have a definite 

 resting period or that they are latent over the winter except as low temperature 

 slows down the development of those produced in the fall. 



Regeneration and Grafting. Like other coelenterates, hydras can replace 

 lost parts. If one is cut transversely and the parts are kept in good conditions, 

 a new basal piece will grow on the one bearing tentacles and a new set of 

 tentacles on the basal piece. Or if a central part of the body is removed it will 

 grow new oral and basal ends in their original relationships. If properly fed, 

 hydras will regain the full size of a lost part within a few days. Regeneration 

 follows a variety of cuts (Fig. 23.9). 



Studies of Hydra. Aristotle knew that coelenterates could sting, thought 

 they looked like plants, and named them zoophyta along with other soft-bodied 

 animals. This name stayed with them for several hundred years. 



Regeneration in animals was first described in hydra. In 1744 Abraham 

 Trembley (1700-1784) made a thorough study of hydras and published a 



Regeneration of bodily portion 



Regeneration of cut anterior end 



Fig. 23.9. Regeneration of hydras. A, successive stages in regeneration of a 

 piece cut from the mid-region of body. B, regrowth of parts of heads — a five- 

 headed animal from original single head. (Courtesy, Fasten: Introduction to Gen- 

 eral Zoology. Boston, Ginn and Co., 1941.) 



