HOMOLOGIES OF ECHIXODERMS. 91 



the sequence in their phases of growth. All we can say at present is 

 that the course of the embryological development of the Spatangoids is 

 such that we can, as it were, re.ad off upon it the sequence of ecliinoidal 

 development since the Jurassic time in the developmental history of 

 some genera of that group. In the one care, however, this development 

 is accomplished in the course of a few years, in the other it stretclies 

 over a comparatively infinite period. We have no data for any such 

 comparisons in the other orders of Echinoderms. 



The case of successive modifications of the ancestral horse, which has 

 so often been brought forward as conclusive regarding the genealogy of 

 the group, although more familiar, is far less complete and much more 

 limited in time than the succession to be traced from the palaeontological 

 evidence of Echini. But while natural selection gives a plausible explana- 

 tion of like problems among Vertebrates, it fails utterly when applied 

 to the majority of the Invertebrates, and we have completely failed 

 thus far to find any causes for their palaGontological develop»nent dif- 

 fering from those acting upon their successive embryological stages at 

 the present day, of which we know absolutely nothing. 



Let us return now to the comparison of the changes undergone by 

 the embryo Echinoderm from its earliest post-Pluteus stages, until the 

 structural features characteristic of the several orders are clearly differ- 

 entiated. The actinal and abactinal surfaces of the embryo Echinoderms 

 in the different orders are, as has been stated, identical ; and it would 

 be impossible to characterize them from early stages immediately follow- 

 ing the resorption of the Piute us, in the same way as from the adult. 

 The abactinal surface consists, in all cases, of a central plate, round which 

 are arranged radial and interradial plates, while the actina^ surfiice is en- 

 tirely occupied by the pentagonal rosette of the water-system, held by the 

 abactinal system as it were in a cup, — a combination which is strictly 

 crinoidal. It is only later that ordinal distinctions appear, but in such 

 succession as to show that the homologies of the several ordei^s as usually 

 understood are not correct. 



In the case of the young Starfish the radial plates of the abactinal system, 

 which form the dorsal part of the arms, gradually extend towards the odge 

 of and down on to the actinal side, enclosing the water-system little by 

 little, and finally, as has been described, covering the ambulacral tube, leav- 

 ing only openings for the passage of the tentacles. This is a st^ge which 



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