ON THE YOUNG STAGES OF ECHINI. 745 



shows the necessity of a careful revision of the whole group of Spatangoids 

 with the data here furnished. Such closely allied genera as Maretia (PL 

 XIX b .f. 7), Spatangus (PL XIX. J. 5), and Macropneustes ; Eupatagus (PI. 

 XV". f. .;) and Metalia (PI. XXI" f. 4) ; Meoma (PI XXII. f. s), Linthia 

 (PI. XIX".f. 7), and Faorina (Pi XIX". f. 4); Agassizia (PI XIX". f. 1), 

 Prenaster, and Periaster; Tripylus (PI XXP. f. 4) and Schizaster (PL 

 XX IIP. f. s); Gualteria, Brissopsis (PL XXI f. 2), and Hemiaster (Pi 

 XXI c .f.i),a,nd many others, both recent and fossil, must be re-examined 

 and critically revised before we can attempt an arrangement of Spatangoids 

 into natural families and genera. 



A general comparison of the changes Echini pass through during their 

 growth, after they have resorbed the pluteus, brings out strikingly the sudden 

 transitions which take place during the development of an individual. For 

 although we can trace the gradual changes by which the young sea-urchin 

 imperceptibly passes from one stage of growth to another, yet there are 

 periods of growth showing such marked differences that they strike us more 

 forcibly, and seem, as it were, to be sudden jumps from one stage of de- 

 velopment to the next, owing to the rapid changes of some of the more 

 prominent points of structure. Very slight changes frequently give a to- 

 tally different facies to a young sea-urchin, .so that the "gap between the 

 extremes will seem very great; but when carefully analyzed the differences 

 disappear, and we are then more struck with the affinities than with the 

 contrasts, which seemed so marked at first glance. 



A few instances will suffice to explain my meaning. To begin with, who 

 would have suspected the genetic relation of the pluteus with the Echinus? 

 The pluteus, an eminently nomadic stage, a scaffolding in which the future 

 sea-urchin plays but a secondary part, and is composed of two open spirals, 

 the one to form eventually the complicated abactinal system ( the interam- 

 bulacral and ambulacral plates), the other to form the water system, and 

 holding between them the digestive cavity and other organs of the pluteus, 

 which as yet appear to have no connection whatever with the spines of the 

 future Echinus. Yet towards the end of the nomadic pluteus life a few hours 

 are sufficient to resorb the whole of the complicated scaffolding, which has 

 been the most striking feature of the Echinoderm. and it passes into some- 

 thing which, it is true, we could hardly recognize as an Echinus, yet has 

 apparently nothing in common with its former condition. In the subse- 

 quent stages contrasts nearly as striking occur. The Cidaris stage (PI. IX. 



