424 COMPARATIVE ZOOLOGY 



2. Alternate Generation 



Sometimes a metamorphosis extending over several 

 generations is required to evolve the perfect animal; 

 " in other words, the parent may find no resemblance to 

 himself in any of his progeny, until he comes down to 

 the great-grandson." Thus, the jellyfish, or medusa, 

 lays eggs which are hatched into larvae resembling 

 Infusoria little transparent oval bodies covered with 

 cilia, by which they swim about for a time till they find 

 a resting place. One of them, for example, becoming 

 fixed, develops rapidly ; it elongates and spreads at the 

 upper end ; a mouth is formed, opening into a digestive 

 cavity ; and tentacles multiply till the mouth is sur- 

 rounded by them. At this stage it resembles a hydra. 

 Then slight wrinkles appear along the body, which 

 grow deeper and deeper, till the animal looks like " a 

 pine cone surmounted by a tuft of tentacles"; and then 

 like a pile of saucers (about a dozen in number) with 

 scalloped edges. Next, the pile breaks up into separate 

 segments, which are, in fact, so many distinct 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 tentacles, 

 circular canals, and other organs exactly like those of 

 the progenitor which laid the original egg (Figs. 20, 



374)- 



Here we see a medusa producing eggs which develop 

 into stationary forms resembling hydras. The hydras 

 then produce not only medusae by budding in the man- 

 ner described, but also other hydras like themselves by 

 budding. All these intermediate forms are transient 

 states of the jellyfish, but the metamorphoses can not be 

 said to occur in the same individual. While a cater- 

 pillar becomes a butterfly, this hydralike individual pro- 



