CHAPTER FIFTH. 



ON THE PLAN OF DEVELOPMENT OF ECHINODERMS. 



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

 plan of our Starfish larva?, 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 apparently bilateral appendages, to show that Muller'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 Muller's observations, concerning the articulate nature of the Echinoderm larvae. 

 The facts already stated respecting the development . of these larva? show that they 

 have only a very remote analogy to some of the larval forms quoted by Huxley, to 

 strengthen his interpretation of the investigations of Miiller. Misled, perhaps, by 

 the names which Miiller 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 Articulates. Huxley has given 

 us no observations of his own bearing upon the subject, but endeavors to justify 

 his assertion by reducing all these forms to one hypothetical type, having an elon- 

 gated 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 larva? of some Annelids. 

 The region in front of the ciliated fringe he calls prcetrochal, and the region behind the 

 fringe posirochal ; 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 regions. He then attempts to prove, further, that there is an intimate con- 

 nection between the point where the young Echinoderm is developed, and the 

 position of the rows of vibratile fringes ; according to him, Starfishes being developed 

 in the postrochal and the Echini in the praetrochal region. Any one who has 

 observed these larva? alive, cannot fail to see that whatever may be the position of 

 these vibratile fringes, the young Echinoderm, whether it be an Echinus, a Starfish, 

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