82 EMBEYOLOGY OF THE STAEFISH. 



of growth, as far as the different classes of the type are concerned. They 

 are all built according to one and the same plan, yet this plan is so car- 

 ried out as to be eminently echinodennoid in one instance, acalephian in 

 another, and polypoidal in a third. In young Echinoderms, as in young 

 CtenophorjB, we find nothing of the remarkable preponderance of certain 

 parts Avhicli gives these young their bilateral appearance in more advanced 

 conditions. Their radiate character is extremely prominent at first, but 

 becomes gradually obscured and hidden under the guise of this bilateral- 

 ity, which is, after all, due only to the excessive development of certain 

 spheromeres as compared with the others. 



The case of these larvce is only an additional example of what we find 

 so often in nature, that a plan of structure which seems to prevail is 

 in reality only an external analogy produced by 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 recall- 

 ing 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.* 



These ti-ansformations are, however, peculiar to the class of Echino- 

 derms ; 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 intermediate individual comes into the cycle. It 

 is the yolk which becomes the larva, the latter being, in its turn, trans- 

 formed into the young Echinoderm. This larva is, in short, an Acalephian 

 larva, reminding us somewhat of the twin individuals of free Hydroids, 

 the Diphyes, though adapted to the mode of development of the Echino- 

 derms. But in the latter we have no intermediate condition corresponding 

 to the Polyp-like Hydroid in Acalephs from which the Medusa? or repro- 

 ductive individuals arise, and in their turn, bring forth the Hydroid again, 

 which completes the cycle by developing another set of Medusae. 



* For a closer comparison of young Ctenophorse and Echinoderm Larvie, see tlie Chapter on Cte- 

 nophoroe, Illustrated Catalogue of the Museum of Comparative Zoology, No. II. 



