224 CASES SIMULATING ALTERNATION OF GENERATIONS. 



the Echinodermata. WTienever, as in tliis class, but one 

 new zooid results from the alternation, it is obvious that 

 the great difficulty of identifying it with the prior structure 

 is removed, and in some of the species of Echinodermata 

 the process has in fact much more the character of a me- 

 tamorphosis than of an " alternation." In the develop- 

 ment, for instance, of the Echinus from its "Pluteus" there 

 is less rejection of the pre-existing structure than occurs in 

 the exuviation of the skin of the Caterpillar, the casting off" 

 of the tail of the Tadpole, or even in the removal of the 

 placenta of the Mammalian foetus ; but in the Starfish — 

 a close ally of the Echinus — though we still have but a 

 single new zooid, our sense of continued identity is obscured, 

 if not destroyed, by the structure in which it originated, re- 

 taining its own vitality, and appearing as a distinct animal, 

 side by side with that which has issued from it. 



After all, it is not metamorphosis which is proved in 

 this way to be akin to alternation, but only that casting off 

 of old structures simultaneously with the formation of new, 

 which, though a common accompaniment of metamorphosis, 

 is far from being its prominent characteristic ; for the de- 

 velopmental changes in which the process essentially con- 

 sists tend much more to the fusion of parts previously dis- 

 tinct, than to the breaking up of prior continuity of struc- 

 ture. This is particularly remarkable in the embryogeny 

 of those species, in which metamorphosis is best marked, 

 as Insects and some of the Crustaceans. On the other 

 hand, it is not in metamorphosis merely, or as a progressive 

 step towards higher organization that we have a casting off 

 of certain parts of the body ; exuviation at least of the 

 tegumentary tissues is a phenomenon, which, continuously 

 or intermittently, and in a greater or less degree, goes on 

 durino; the whole lifetime of the individual in almost all 

 species, and is in fact absolutely the same in kind with that 

 constant molecular renewal of the body, in which the very 



