90 HOMOLOGIES OF ECHINODEEMS 



derms in the older geological deposits, that, if any development from 

 one order to another has taken place, it must have been during much 

 earlier geological periods. As far as we now know, palaeontology throws 

 no light whatever upon such a transition, however possible it may seem 

 from embryological data. Starfishes, Ophiurans, Echini, and Crinoids ex- 

 isted in the oldest-known Echinodermoid fauna, having all the typical 

 features of Echinoderms of our day, or only so for modified as to be 

 readily homologized with them. If there has been such a thing as a 

 single ancestral Echinoderm, his primordial descendants early assumed 

 different lines of development diverging to a great degree, and retain- 

 ing their characteristics from the earliest-known geological period. This 

 at least appears to be the case with Starfishes * and Ophiurans ; while 

 the different groups of Crinoids which have appeared and vanished are 

 numerous as compared to those of the other orders. 



The Echini again continued to develop until the secondary period with 

 very little modification, and only after the Jurassic period did the marked 

 changes begin through which they subsequently pass ; changes fully equal- 

 ling those of the Crinoids in their earlier geological history. t And yet, great 

 as these changes have undoubtedly been, were we to measure them sim- 

 ply by pateontological evidence, we must remember that they are not 

 greater in degree than the changes known to take place among the 

 Echini of the present day during their embryological development. But 

 while the successive appearance of the great types of Echini in geologi- 

 cal time — in other words, their paliBontological development — is in the 

 strictest harmony with what we know of their embryological develop- 

 ment,t we as certainly know nothing whatever of the causes which have 

 brought about their sequence in time, in such striking agreement with 



* The attempt made by G. O. Sars to prove Brisinga to be the living representative of the palaeo- 

 zoic Starfishes seems to be very far-fetched, and I must acknowledge I have been unable to see 

 any such radical difference between Brisinga and ordinary Starfishes (Solaster, Crossaster, and Pycno- 

 podia, for instance) as Sars insists upon in liis Memoir on Brisinga. The type of Starfishes, as I have 

 already shown, has been remarkably persistent from the earliest geological periods to the present 

 day, and there is no indication that the Starfishes now living have undergone such changes as to 

 make the agreement of Brisinga or other genera with the older forms a matter worthy of especial 

 notice as survivals or representatives of the earlier types. 



See also the view taken by Liitken in regard to the affinities of Protaster in his Ophiur. Add. III. ; 

 he does not agree with the view taken by Sars. 



t See A. Agassiz's Revision of the Echini, Part IV. 



J See Part IV., Revision of the Echini, by A. Agassiz. 



