DEVELOPMENT. 



213 



in number) with scalloped edges. Next, the pile breaks 

 up into separate segments, which are, in fact, so many dis- 

 tinct animals; and each turning over as it is set free, so as 

 to bring the mouth below, develops into an adult Medusa, 

 becoming more and more convex, and furnished with ten- 

 tacles, circular canals, and other organs exactly like those of 

 the progenitor which laid the original egg (Figs. 178, 195). 

 Here we see a Medusa producing eggs which develop 

 into stationary forms resembling Plydras. The Hydras 



Fi&. ITS — Alternate Generation : a, 6, c, ova of an Acaleph (Chrysaora) ; d, e,/, Hy- 

 dras ; g, ft, Hydras with constrictions ; i, Hydra undergoing lissiou ; k, one of the- 

 separated segments, a free Medusa. 



then produce not only Medusae by budding in the manner- 

 desci'ibed, but also other Hydras like themselves by bud- 

 ding. All these intermediate forms are transient states- 

 of the Jelly-fish, but the metamorphoses cannot be said to 

 occur in the same individual. While a Caterpillar heoomes' 

 a Butterfly, this Hydra-like individual ^Of^wces a number 

 of Medusae. Alternate generation is, then, an alternation 

 of asexual and sexual methods of reproduction, one or- 

 more generations produced from buds being followed by 

 a single generation produced from eggs. Often, as in 

 the fresh-wkter Hydra, the two kinds of generations are 

 alike in appearance. The process is as widespread as 

 asexual reproduction, being found mostly in Sponges, 

 Ccelenterates, and Worms. It is also found in certain 



