114 ESSAY ON CLASSIFICATION. Part I. 



geological formations; first, it recalls the Cistoicls of the palaeozoic rocks, -svliich are 

 represented in its simple spha^roidal head, next the few-plated Platycrinoids of the 

 Carljoniferous period, next the Pentacrinoids of the Lias and Oolithe, with their whorls 

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

 as the 2^1'omhient type of the famil}^, in the present period. The investigations of 

 Miiller upon the larvae of all the families of living Asterioids and Echmoids enable 

 us to extend these comparisons to the higher Echinoderms also. The first point 

 Avhich strikes the observers in the facts ascertained by Miiller, is the extraordinary 

 similarity of so many larvte, 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 reproducing their typical peculiarities. 

 It is next very remarkable, that the more advanced larval state of Echinoids and 

 Spatangoids should continue to show such great similarity, that a young Amphidetus 

 hardly difters from a young Echinus.^ Finally, 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 representa- 

 tives belong to the genera Diadema and Cidaris, next come true Echinoids, later 

 only Spatangoids. When the embryology of the Clypeastroids is known, it Avill, 

 no doubt, aftbrd other links to connect a larger number of the memljers of this 

 series. 



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

 affords but a few data for such comparisons. It is, nevertheless, worthy of remark, 

 that while the young LamclUbranchiata are still in their embryonic stage of growth, 

 they resemble, externally at least, Brachiopods^ more than their own parents, and 

 the young shells of all Gasteropods* knowTi in their embryonic stage of growth, 

 being all holostomate, recall the oldest types of that class. Unfortunately, nothing 

 is yet known of the embryology of the Chambered Cephalopoda, which are the only 

 ones found in the older geological formations, and the changes wliich the shield of 

 the Dibranchiata undergoes have not yet been observed, so that no comparisons can 

 be estabhshed between them and the Belemnites and other representatives of this 

 order in the middle and more recent geological ages. 



Respecting Worms, our knowledge of the fossils is too fragmentary to lead to 

 any conclusion, even should our information of the embryology of these animals 



' Compare J. Miiller's 1st paper, pi. III., with * Se« the works, q. a., p. 73, note 1. 



pis. IV.-VII., and with pis. VI. and VII., 4th paper. ^ See the works, q. a., p. 73, note 2, especially 



^ Agassiz, (L.,) Twelve Lectures, q. a., etc. p. 25. those relating to Nudibranchiata. 



