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. 185c) 

 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 yj, )|// // 



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 vathev than quali- 

 tative increase. One individual is 



Fig. 185. The Colonial hydroid {Bougainvillea^ 

 ^, 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; w, radial canal; 

 «, sense organ; so, tentacle; b and c, after 

 Allman. 





