HYDROZOA 



215 



the development, which usually includes an alternation of generations. 

 From an egg of a medusa a small ciliated embryo {planula) escapes, 

 which becomes attached, develops mouth and tentacles, and, by budding, 

 produces a hy droid colony. This colony lacks sexual organs. By budding 



Fig. 178. — Diagram of sections of (.4) a poh-p and (B) a medusa. cJ, ectoderm; 

 ek', of exumbrella; ek~, of subumbrella; ek^, of manubrium; el, endoderm (cathamnal) 

 layer arising from ol^Literation of digestive space; en, entoderm; r, ring canal; 5, sub- 

 umbrella; (, tentacles; v, velum; -v, supporting layer (gelatinous in B). 



it produces sexual individuals (medusa?) which separate and swim away. 

 Since polyp and medusae are morphologically comparable, before the 

 escape of the medusa the colony is polymorphic, consisting of individuals 

 (hydranths) which reproduce only asexually and of others which have 

 taken over the sexual reproduction (medusa;). Hence alternation of 



Fig. 179. — Comparison of a medusa and a sporosac (orig.). A, fully developed 

 medusa; B, medusa with the manubrium closed, still attached to the blastostyle; C, 

 medusa reduced to a simnle manubrium (sporosac); D, last stage, eggs being produced 

 in the body wall {Hydra). 



generations has arisen here from a division of labor or polymorphism 

 of individuals originally of equivalent value, in which some individuals 

 (the sexual) have separated and acquired a peculiar structure. 



While alternation of generation has arisen from polymorphism, it can 

 again produce it. This occurs when the medusa;, instead of separating, 



