Il6 COLLEGE ZOOLOGY 



becomes detached and leads a separate existence. Sometimes 

 the bud may begin to form other buds before it becomes de- 

 tached from the parent animal. In this way a sort of hydroid 

 colony is produced resembling that of certain marine ccelenterates 

 like Obelia (Fig. 73). Fission is less common. The distal end 

 of the animal divides first; then the body slowly splits down the 

 center, the halves finally separating when the basal disc is sev- 

 ered (Fig. 71). Hydras have also been found which bore buds 

 reproducing in this manner. This method of multiplication must, 



however, be rare, since it is so 

 seldom seen. Transverse fission 

 has also been reported. 



The processes concerned in 

 sexual reproduction are the pro- 

 duction of spermatozoa and 

 eggs, the fertilization of the egg, 

 the development and hatching 

 of the embryo, and the growth 

 of the young larva. The sper- 

 - I V I Hydm /A? rod i; " d r g by matozoa arise in the testis from 



longitudinal nssion. (After Koehtz in 



Zooi. Anz.) ectodermal interstitial cells 



(Fig. 65, y.t.}; they develop in 



long cysts (Fig. 65, m.t.) through the end of which they escape 

 into the surrounding water. The eggs arise in the ovary from 

 ectodermal interstitial cells (Fig. 65, y.e.). Usually only one 

 egg develops in a single ovary. When a certain period of 

 growth is reached, two polar bodies (Fig. 65, p.b.) are given off by 

 the egg, which is then said to be mature (Fig. 65, m.e.). Fer- 

 tilization occurs usually within two hours after the polar bodies 

 have been formed. 



The cleavage of the egg is total and almost equal, a blastula 

 (Fig. 65, bl) being formed with a distinct cavity, the blastoccel. 

 A solid gastrula-like structure (Fig. 65, g) is produced by the 

 filling up of the blastoccel with cells budded off from the blas- 

 tula wall. The outer cells may be called ectoderm and the inner 



