ON THE EMBRYOLOGY OF ECHINODERMS. 15 



young Mellita a strong resemblance to li/icope MicheVun Ag. and E( hinoglycus (Lobo- 

 phora Stokesii Ag.). The anterior ambulacral lunule is last formed. The posterior 

 lunules then close, and had we not the anterior notches we should have an Amphiope. 



the three anterior lunules, gives to the young 



The last step, which is the closing of the three anterior lunules, 

 Mellita the general aspect of the adult ; it has then attained a diameter of three quarters 

 of an inch. In the young of Encope the same succession of changes is observed ; at first 

 they are circular, with simple notches in place of the posterior lunules ; the anterior 

 lunules are then formed, the posterior ones close, and, last of all, the anterior ones. 

 In the early stages it is a Lobophora, then an Echinodiscus (Lobofora bifora Ag.). 



In the young of Echinarachnius the changes of form are equally important; small 

 specimens measuring not more than one fourteenth of an inch are very elliptical in 

 outline, the longer axis being twice as great as the shorter ; the test is quite convex, 

 resembling strikingly Echinoeyamus and Fibularia ; the anus is placed on the abac- 

 tinal side, and not on the, edge of the test, as in the adult. This shape the young soon 

 loses to attain a more circular outline, becoming flattened, while the anus is pushed 

 nearer the edge. In specimens in which the longer axis measured one fifth of an inch, 

 the test is quite flat, and the anus is placed on the edge of the disk. The outline is still 



different from that of the adult, the longer axis being the 



these young 



mens, while in the adult it is the axis at right angles to the odd ambulacrum 



o" w **"& 



which is the longest. 



Erom these data important results have been drawn for the classification of Echi- 

 noids * All Echinoids pass, in their early stages, through a condition which recalls to 

 us the first Echinoids which made their appearance in geological ages. We should, 

 then, on embryological grounds, place true Echini lowest, then the Clypeastroids, next 

 the Echinolamps, and finally the Spatangoids. The true Echini are an embryological 

 suborder ; the Clypeastroids recall to us, in their young stages only, this lowest sub- 

 order. The Echinolamps have lost the teeth which characterize the first suborders, 

 and they show a tendency to develop an upper and a lower side, a front and a behind, 

 which is carried to its extreme among Echinoids in the Spatangoids, the highest sub- 

 order, the young of which cannot be distinguished from those of the lower suborders 

 at the time when they are provided with few stout spines. There are not a sufficient 

 number of living representatives of Spatangoids and Echinolamps of which we know 

 the development to go into more detail with respect to the standing of minor groups. 

 In Clypeastroids what has preceded seems to show that we must place Fibularia and 



* L. Agassiz. Essay on Classification 



