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vous system). That the ambulacral skeleton in the Asterides, in spite of its habitual resem- 

 blance, has no analogy whatever with the vertebral column in the Vertebrata, but properly 

 belongs to the cuticular skeleton - - is satisfactorily ascertained, and is clearly evidenced 

 (inter alia) by its development being perfectly conformable to that of the cuticular skeleton. 

 We have not yet touched on a point which, just where there is a question of phylo- 

 genetic relations, must occupy a very important place, and will often alone be able to give 

 us the most certain and significant indication in this respect; I mean the history of deve- 

 lopment. We shall find also in considering this important part of the natural history of 

 the Echinoderms, an essential support for the theory above noticed; at the same time as 

 many hitherto quite unintelligible points connected with the subject will, by help of this 

 theory, find a natural explanation. As is well known, most Echinoderms go through a most 

 remarkable and peculiar metamorphosis, which has first been made the subject of particular 

 and minute research by the celebrated German naturalist Johannes Miiller. From the egg 

 there proceeds a creature of most romantic appearance, furnished with various lobes and 

 processes, swimming away by means of sinuous bands of cilia, and having no resemblance 

 whatever to the respective Echinoderin, being a perfectly bilaterally symmetrical animal, 

 which according to its whole structure would most naturally be referred to the group of 

 Vermes, and which also exhibits a striking resemblance to the so-called larvae of certain 

 Nemertina and Gephyrea. Only at a later period, there is laid, in a limited part of the 

 interior of this worm-like animal, the foundation of the future echinoderm; and as the latter 

 is developed, the original so-called larva-body shrivels up little by little or becomes resor- 

 bed. There is much to forbid considering this peculiar process of development as a real 

 metamorphosis. It is indeed usual for the whole larva-body to be resorbed, or as it were 

 taken up in the formation of the Echinoderm; but we have still instances of this not being 

 always the case. Thus the Bipinnaria-larva continues to swim about even after the young 

 star-fish has detached itself; and it may even be imagined possible that such a larva can 

 again give origin to a new star-fish. This instance alone is sufficient to distinguish very 

 sharply the development of the Echinoderms from the ordinary development by metamor- 

 phosis. We must on the contrary bring it under the great law of alternate generation, as 

 a peculiar modification of the same, chiefly characterised by the cycle of generation being, 

 as far as we yet know, only represented by 2 Bionta. It is this form of alternate gene- 

 , ration which has been called by Hackel, Metagenesis successiva. We have consequently in 

 the peculiar so-called Echinoderm-larva properly speaking not an incomplete stage of deve- 

 lopment, but one of the 2 fully developed alternating generations, namely the non sexual 

 generation. The apparently paradoxical and unintelligible fact that the 2 Bionta in the same 

 cycle of generation are constructed in totally different fundamental forms, one in the racliary 

 and the other in the bilaterally symmetrical (eudipleurous) form, can only find its natural 

 explanation, if we consider the developed Echinoderm as an individual of a higher order, or 

 a colony, the apparently corresponding parts or antimera of which represent the original 



