FIFTH CHAPTER. 



ON THE PLAN OF DEVELOPMENT OF ECHINODERM^. 



We have constantly insisted, during the whole of this Memoir, upon 

 the radiate plan of our Starfish larvae in their different stages of growth. 

 We have, however, seen that this radiate plan of structure, at certain 

 periods of their existence, is so far hidden by the apparent bilateral 

 arrangement of the locomotive appendages as readily to escape notice. 

 We have also had occasion, in discussing the development of these appar- 

 ently bilateral appendages, to show that Miiller's views of the bilateral 

 nature of these larvae were founded upon mistaken analogies. It now 

 remains for us to examine, somewhat in detail, the theory put forth by 

 Huxley, in his review of Midler's observations, concerning the articulate 

 nature of the Echinoderm larvae. The facts already stated respecting 

 the development of these larvae show that they have only a very remote 

 analogy to some of the larval forms, quoted by Huxley in order to strength- 

 en his interpretation of the investigations of Muller. Misled, perhaps, by 

 the names which MUUer has given to some of these larvae (" Wurmformige 

 Larven"), he has allowed this analogy to influence him so far that he revives 

 the old opinion of Oken, and refers the Echinoderms to the type of Articu- 

 lates. [See my Memoir on Balanoglossus, for a later review of the views 

 of Huxley, Haeckel, and others, who have urged the affinities of Echino- 

 derms with worms.] Huxley has given us ]io observations of his own, 

 bearing upon the subject, but endeavors to justify his assertion by redu- 

 cing all these forms to one hypothetical type, having an elongated form, 

 a straight intestine, with the mouth at one extremity, the anus at the 

 other, and girded by a circular ciliated fringe, just like the larvre of some 

 Annelids. The region in front of the ciliated fringe he calls praiiochal, 

 and the region behind the fringe postrochal ; and then, by an ingenious 

 process, he shows how all these different forms might be produced by 



the greater or less development of one or other of these region. He 



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