Chap.V. plan of development of ECHINODERMS. 65 



is, after all, clue only to the excessive development of certain spheromeres as com- 

 pared with the others. 



The case of these larva? is only an additional example of what we find so 

 often in nature, that one plan of structure apparently prevails, while, in reality, 

 it is only an external analogy, obtaining a great predominance in certain parts, 

 but subservient to the primary plan, even though the latter be perceived only on 

 closer examination. This view solves a question which has hitherto perplexed all 

 investigators of this subject, — viz.: how it was possible that a larva, which has 

 always been considered as bilateral, should produce a radiate animal by a process 

 of internal gemmation. It is, indeed, a bilateral larva, but built upon a radiate 

 plan; a larva recalling a lower class of this branch of the animal kingdom, an 

 acalephian larva giving rise to an Echinoderm, which, from its very beginning, 

 is a radiate animal, having all its spheromeres developed at the same time, and 

 equally. 1 



These transformations are, however, peculiar to the class of Echinoderms; they 

 constitute neither a metamorphosis, nor a case of alternate generation. The egg 

 becomes the embryo larva, nothing essential is lost during the process, no intermedi- 

 ate individual comes into the cycle. It is the yolk which becomes the larva, the 

 latter being, in its turn, transformed into the young Echinoderm. This larva is, 

 in short, an acalephian larva, reminding us somewhat of the twin individuals of free 

 Hydroids, though adapted to the mode of development of the Echinoderms. But, in 

 the latter, we have no intermediate condition corresponding to the Polyp-like Hydroid 

 in Acalephs, from which the Medusa? or reproductive individuals arise, and, in their 

 turn, bring forth the Hydroid again, which completes the cycle by developing another 

 set of Medusae. 



If the views here taken of the plan of development of Echinoderms be correct, 

 they introduce a new set of facts respecting their affinities with the Polyps and 

 Acalephs, which cannot fail to have an important bearing on the question of the 

 separation of the Echinoderms as a distinct type from the two latter groups. The 

 Echinoderm plutean form, with its mouth, stomach, intestine, with the water-system, 

 originally forming a part of the digestive cavity, bears, as it seems to me, the same 

 relation to the Ctenophoraa which the Hydroid Polyps hold to the true Polyps. The 

 Ctenophora? may be considered, as it were, the prophetic type of the Echinoderms, 

 as the Polyps are the prophetic type of Acalephs. We have in the Ctenophorse a 

 digestive cavity, from which branches the water-system, and that peculiar funnel, 

 opening outwards, through which the fecal matters of the Ctenophora? are discharged, 



1 For a closer comparison of young Ctenophorse logue of the Museum of Comparative Zoology, 

 and Echinoderm Larvas, see the Illustrated Cata- No. II., now in press. 

 vol. v. 9 



