16 



ON THE EMBRYOLOGT OF ECHINODEEMS. 



Echinocyamus lowest; then the rounded Clypeasters, Clypeaster and Khaphydocly- 

 pus, in which we have the floor connected by but few supports ; then forms such 

 as Laganum ; then the circular Scutellse, such as Arachnoides, in which the anus is 

 supramarginal ; then Echinarachnius, Dendraster, Scaphechinus ; then the genera 

 Lobophora, Echinodiscus, Echinoglycus, Encope, Rotula, and Mellita. 



Adapting in a similar way the observations of the different stages of our young 

 Echinus given in this Memoir to the true Echinoids, Ave should for similar reasons 

 place lowest the family of Cidaridte; next, the Diadematidaj ; then that peculiarly 

 embryonic family, the Echinometrada;, in which the unwinding of the pentagons leaves 

 the Sea-urchin with oblique axes ; then Sea-urchins with few larger spines, such as 

 the Echinocidaridee and Heliocidaridse. In all these families, with the exception of the 

 HeliocidaridfE and Echinometradse, the ambulacral system is particularly simple. We 

 next have those Sea-urchins with a more complicated ambulacral system, in which the 

 tubercles become numerous and are not arranged in such regular vertical rows as in 

 the true Echinidse, Toxopneustes, and the like ; then the Hipponoidae, and the like, in 

 which the development of the ambulacral system reaches its greatest complication, 

 in which the spines are exceedingly fine, and in many genera (such as Salmacis 

 and Mespilia) resemble more what we find in the Clypeastroids ; passing gradually 

 through forms such as Boletia, Tripneustes, Hipponoe, Salmacis, Mespilia, to take 

 their greatest degree of complication, both in the ambulacral and interambulacral 

 regions, in Holopneustes. 



The correspondence between the embryological development and the order of succes- 

 sion of Echinoids in geological times is so striking, that it may not be out of place to 

 show some of the principal points of agreement.* The number of fossil Echinoids 

 known is so great, that, when we have as large a number of embryonic forms for each 

 species to compare with them as are here given for a couple of species, we shall not 

 fail to draw most important conclusions for our knowledge of the classification of these 

 animals. 



The earliest Echinoids,-|- which make their appearance with the Trias, are without 

 exception Sea-urchins, belonging to families which have eminently embryonic charac- 



• See Agassiz, L. Catalogue Raisonnd . . . des Echinodermes . . . An. ties Sc. Nat., 184G - 47. 



t I omit intentionally the presence of such forms as Paljechinus in the Silurian, Eocidaris in the Devonian, 

 and the many Ai-chawciilaris and Melonites of the Carboniferous, until a comparative study of these forms with 

 the younger stages of Comatula has been made. I am convinced from wliat I know of the embryology of 

 Echinoderms, that they are only synthetic and prophetic Crinoids, and have therefore nothing to do with our 

 present subject. 



