16 



ON THE EMBRYOLOGY OF ECHINODERMS. 



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

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

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

 supramarginal ; then Echinarachnius, Dcndraster, 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, we should for similar 



reasons 



ice lowest the family of Cidaridse; next, the Diadematidre ; then that peculiarly 

 embryonic family, the Echinometradre, in which the unwinding of the pentagons leaves 

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

 the Echinocidaridae. and Heliocidaridae. In all these families, with the exception of the 

 HeliocidaridaB and Echinometradae, the ambulacral system is particularly simple. We 

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

 tubercles become numerous and are not arranged in such regular vertical 

 the true Echinidee, Toxopneustes, and the like ; then the Hipponoidic and I 



rows as in 



wh.ch 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 Mespil.a) resemble more what we find in the Clypeastroid S ; passing gradually 

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

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

 regions, m Holopneustes. 



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

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

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

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

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

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



animals. 



exc^l" T l E f n °l dS,t WWCh make th6ir appearanCC With the T -s- •» -*out 



exception Sea-urchms, belonging to families which hav 



eminently embrj 



* 



t 



See Agassiz, L. CataWue 



47. 



Archeeocidari 



.be younger stages of Coma,„,a hm J?™ VT • T" * "" *"*• -°7 of these forms ,v,«h 



Eehinoderms that thev »r» t , convmced from what I know of the embryology of 



present snhjecl ' ' ^^ ""* **»>* °*»«* «* «-« "-afore nothing ,o do with our 



