320 



ESSENTIALS OF ZOOLOGY 



extension of the enteron extends into the bud, which is essentially 

 an outgrowth of the body wall. Tentacles appear as outpushings of 

 ectoderm and endoderm, and in the terminal position a mouth is 

 developed. After the bud has attained some size, a constriction oc- 

 curs between it and the parent, finally separating the two to form 

 a free individual. 



Sexual Reproduction. — During the summer and fall particularly, 

 hydra reproduces sexually. This involves the production, matura- 

 tion, and union of germ cells. Testes may appear first and ovaries 

 later on the same individual or both gonads may be present at the 



Embryo 



HydrAr <§exu&l i^production 



Fig. 114. 



-Methods of reproduction in hydra. (Courtesy of General Biological 



Supply House.) 



same time in which case self-fertilization is possible. As a rule, 

 these animals are hermaphroditic or monoecious as suggested before, 

 but it has been reported that individuals of separate sex (dioecious) 

 have been found. The germ cells or gametes develop from inter- 

 stitial cells which accumulate at a certain place between the ectoderm 

 and endoderm, where they multiply by division to form oogonia in 

 the female gonad and spermatogonia in the male. All phases of 

 maturation (gametogenesis) may be observed in the testis and ovary. 

 The testis produces large numbers of motile spermatozoa, which when 

 mature emerge periodically from an opening in the tip of the testis 

 and are discharged free in the water. In the ovary a single egg 



