THE LIFE CYCLE 



331 



there the sexual and asexual 

 phases of the life cycle respec- 

 t i v e 1 y . The free-swimming 

 medusa (jelly fish, fig. 18-50) 

 produces the eggs and sperms 

 and liberates them in the sea. 

 These after fertilization develop 

 into sessile hydranths (more or 

 less similar to the common hy- 

 dra) , which in turn develop me- 

 dusae by 

 various 

 modes of 

 asexual 

 budding. 



II. SPECIAL METHODS -OF ASEXUAL 

 REPRODUCTION. 



Sexual reproduction results in the 

 main in a qualitative increase in a 

 species. Without segregation it 

 tends toward reducing all the forms 

 to a common level, but with segrega- 

 tion, whether external or internal, it 

 is a potent means of effecting 

 species change. -Asexual reproduc- 

 tion is quantitative rather than quali- 

 tative .increase. One individual is 



FIG. 185. The Colonial hydroid (Bougainvillea^ 

 a, the form of a small colony; b, a bit from the 

 tip of one of the branches; w, tentacles of a 

 single hydranth ; x, y, z, stages in the develop- 

 ment of the buds (medusae), c, The fully 

 formed and free swimming medusa (jelly fish) 

 k, the body (manubrium) with, the mouth at 

 its tip;/, the surrounding bell; m, radial canal; 

 H, sense organs; o, tentacle; b and c, after 

 Allman. 



