CHAPTER XXIII. 



THE ENTEROPNEUSTA, PTEROBRANCHIA, POLYZOA, BRACHIO- 

 PODA, PHORONIDA AND CH^.TOGNATHA. 



IV. The Enteropneusta. (Figs. 296, 297, 298.) 



The resemblance between the larvae of the enteropneusta and echinoderms 

 has been frequently emphasized, and it is generally assumed that it indicates a 

 common origin for both classes in some worm-like ancestors. We shall show that 

 the resemblance between these larvae is largely superficial; that between them there 

 are underlying differences, so great as to preclude a direct genetic relation of one 

 to the other, and that neither one nor the other resembles the larva of annelids. 



The enteropneusta have also been compared with the vertebrates, because of 

 their aboral nerve cord, perforated gill sacs, "enteric" coelom, and "notochord." 

 The first two points are of real significance, and together with other evidence, 

 indicate an intimate relation with Amphioxus, tunicates, and other acraniates, but 

 not with vertebrates. The last two points, the " enteric coelom " and " notochord" 

 have a purely artificial value that has been created by a false interpretation of the 

 early processes of development, for the " archenteron " and the "gastrula" that 

 lie at the bottom of the whole system, have no existence. 



The Enteropneusta and the Echinoderms. — The early development of the 

 enteropneusta is very significant. We have already seen that when the gastrula is 

 retained in an essentially unmodified condition, and has the significance originally 

 attached to it, it always gives rise to the endoderm only, its cavity remains as the 

 cavity of the enteron, and its external opening remains as the neurostoma; or the 

 latter is formed at the point where the gastrula opening, or blastopore, closed. 

 These conditions prevail in the annelids and molluscs. Wherever the two inner 

 germ layers arise from a caudal infolding that gives rise to both mesoderm and 

 endoderm, to mesocoele and endoccele, and that in closing gives rise to the anus, not 

 to the mouth, we are dealing with a highly modified process of development that is 

 only remotely comparable with gastrulation, and the presence of which is prima 

 facie evidence that the animals in which it occurs are descended from arthropod 

 ancestors. 



It is the latter type of development that is characteristic of the enteropneusta, 

 hence it is apparent at the very outset that their simple structure is not a primitive 

 condition but a secondary one, and that the location of the functional mouth on 

 the haemal, instead of the neural surface, the formation of an ento-mesoccele in 

 place of a gastrula, and the presence of a telopore in place of a blastopore, de- 

 finitely excludes the enteropneusta from both the molluscs and annelids, and 

 places them among the descendants of the arthropods. 



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