THE DEVELOPMENT OF ORGANISMS 769 



of Generations, which is oi wide occurrence both among plants and 

 animals. Many of the fixed, vegetative, plant-like, asexual zoophj'tes 

 or hydroid colonies, which grow on stones and shells and seaweeds 

 in shallow shore- waters, bud off swimming-bells or medusoids in 

 the summer months. These beautiful transparent bells swim in the 

 open waters, and are sexual. Their fertilised eggs develop into 

 embryos, which become free larvae, which soon settle down as 

 polyps, and form new arborescent colonies by budding. This is 

 typical alternation of generations, of the type that is technically 

 called metagenesis. The asexual hydroids bud off sexual medusoids, 

 whose fertilised egg-cells give rise to hydroids again (Fig. 125). 



Fig. 126. 



The Strobila Phase in the Life-History of a Jellyfish (Aurelia). It consists of 

 a vertical series of horizontal buds, often compared to a pile of saucers, 

 each of which in turn is separated off as an ephyra or young jellyfish. 

 E, uppermost and oldest scyphomedusa or disc; S, a sense-organ in one 

 of the bifid marginal lobes; SEC, second youngest disc; HT, hydra- 

 tuba, the polyp-like individual from which the discs are successively 

 budded off. 



To take a supeificially very different case, the common medusa 

 or jellyfish (Aurelia) gives rise to egg-cells or sperm-cells, and the 

 fertilised egg-cell develops into a free-swimming larva. This settles 

 as a polyp-like form, the hydra-tuba, and this, by a kind of budding, 

 gives origin to little saucer-like discs which grow into medusae. In 

 this case the sequence is : Medusa, sex-cells, free larva, fixed asexual 

 polyp, budding, young medusa (Fig. 126). 



Now it is an interesting fact that some medusae, like the lumines- 

 cent Pelagia, common on the high seas, give rise sexually to other 

 medusae, without the interpolation of a fixed polypoid stage. Simi- 

 larly there are many swimming-bells or medusoids (anatomically 

 very different from medusae) whose fertilised egg-cells also give 



VOL. II D 



