172 ESSAY ON CLASSIFICATION. 



exactly with what is known of the successive appearance 

 of Echinoicls in past ages : a their earliest representatives 

 belong to the genera Diadema and Cidaris, next come 

 true Echinoids, later only Spatangoids. When the em- 

 bryology of the Clypeastroids is known, it will, no doubt, 

 afford other links to connect a larger number of the mem- 

 bers of this series. 



What is known of the embryology of Acephala, Gaste- 

 ropoda, and Cephalopoda, affords but a few data for such 

 comparisons. It is, nevertheless, worthy of remark, that, 

 while the young Lamellibranchiata are still in their em- 

 bryonic stage of growth, they resemble Brachiopods, 2 

 externally at least, more than their own parents, and the 

 young shells of all Gasteropods 3 known in their embry- 

 onic 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 for- 

 mations, and the changes which the shield of the Dibran- 

 chiata undergoes have not been observed, so that no 

 comparisons can be established between them and the 

 Belemnites and other representatives of this order in the 

 middle and more recent geological ages. 



Eespecting Worms, our knowledge of the fossils is too 

 fragmentary to lead to any conclusion, even should our 

 information of the embryology of these animals be suffi- 

 cient, as a basis for similar comparisons. The class of 

 Crustacea, on the contrary, is very instructive in this 

 respect ; but, to trace our comparisons through the whole 

 scries, it is necessary that we should consider simul- 



1 ACASSIZ (L.,) Twelve Lectures, 3 See the works, q. a., p. 110, n. 1, 

 q. a., etc., p. 25. especially those relating toNutlibrau- 



2 See the works, q. a., p. 110, n 1. chiata. 



