138 ESSAY ON CLASSIFICATION. 



In the case of the Medusae proper, 1 the parent lays eggs 

 from which polyplike individuals originate ; but here these 

 individuals separate by transverse constrictions into a num- 

 ber of disks, and every one of these undergoes a succession 

 of changes, which end in the production of as many indi- 

 viduals, each identical with the parent, and capable in its 

 turn of laying eggs, (some, however, being males, and 

 others females.) But the polyplike individuals born from 

 eggs may also multiply by budding, and each bud undergo 

 the same changes as the first, the base of which does not 

 die, but is also capable of growing up again and of re- 

 peating the same process. 



In other classes, other phenomena of a similar character 

 have been observed, which bear a similar explanation. 

 J. Muller 2 has most fully illustrated the alternate genera- 

 tions of the Echinoderms ; Chamisso, Steenstrup, Eschricht, 

 Krohn, and Sars, those of the Salpse ; 3 von Siebold, Steen- 

 strup, and others, those of certain Intestinal Worms. 4 



This alternate generation differs essentially from meta- 

 morphosis, though some writers have attempted to identify 

 these two processes. In metamorphosis, as observed among 

 Insects, the individual born from an egg goes on under- 

 going change after change in direct and immediate suc- 

 cession, until it has reached its final transformation ; but, 

 however different it may be at different periods of its life, 

 it is always one and the same individual. In alternate 

 generations, the individual born from an egg never as- 

 sumes through a succession of transformations the cha- 

 racter of its parent, but produces, either by internal or 

 external budding or by division, a number sometimes 



1 See SIEBOLD, and SARS, q. a., p. 3 See the works, q. a., page 108, 

 103, note 3. note 3. 



2 MULLER, (J.,) Ueber den allge- 4 See the works, q. a., p. 115, note 

 meinen Plan, etc., q. a., p. 105, note 1. 1, and 116, note 1. 



