112 ESSAY ON CLASSIFICATION 



No class, as yet, affords a more complete and more beautiful evi- 

 dence of the correspondence of their embryonic changes with the 

 successive appearance of their representatives in past ages than the 

 Echinoderms, thanks to the extensive and patient investigations of 

 J. Miiller upon the metamorphoses of these animals. Prior to the 

 publication of his papers the metamorphosis of the European Coma- 

 tula alone was known. (See Sect, xviii.) This had already shown that 

 the early stages of growth of this Echinoderm exemplify the pedun- 

 cated Crinoids of past ages. I have myself seen further that the suc- 

 cessive stages of the embryonic growth of Comatula typify, as it were, 

 the principal forms of Crinoids which characterize the successive 

 geological formations; first, it recalls the Cistoids of the palaeozoic 

 rocks, which are represented in its simple sphasroidal head; next the 

 few-plated Platycrinoids of the Carboniferous period; next the Pen- 

 tacrinoids of the Lias and Oolite, with their whorls of cirrhi; and 

 finally, when freed from its stem, it stands as the highest Crinoid, as 

 the prominent type of the family in the present period. The investi- 

 gations of Miiller upon the larvie of all the families of living Asteri- 

 oids and Echinoids enable us to extend these comparisons to the 

 higher Echinoderms also. The first point which strikes the observers 

 in the facts ascertained by Miiller is the extraordinary similarity of 

 so many larvse, of such different orders and different families as the 

 Ophiuroids and Asterioids, the Echinoids proper and the Spatangoids, 

 and even the Holothurioids, all of which end of course in reproduc- 

 ing their typical peculiarities. It is next very remarkable that the 

 more advanced larval state of Echinoids and Spatangoids should con- 

 tinue to show such great similarity, that a young Amphidetus hardly 

 differs from a young Echinus. Einally, not to extend these remarks 

 too far, I would only add that these young Echinoids (Spatangus, as 

 well as Echinus proper) have rather a general resemblance to Cidaris 

 on account of their large spines, than to Echinus proper. Now these 

 facts agree exactly with what is known of the successive appearance 

 of Echinoids in past ages; their earliest representatives belong to the 

 genera Diadema and Cidaris, next come true Echinoids, later only 

 Spatangoids. When the embryology of the Clypeastroids is known, 

 it will no doubt afford other links to connect a larger number of the 

 members of this series. 



What is known of the embryology of Acephala, Gasteropoda, and 



