ON THE YOUNG STAGES OF ECHINI. 



To complete the history of the development of Echini just given, I add 

 here the main portions of the sketch on young Echini given in my Pre- 

 liminary Report on the Florida Deep Sea Echini, with references to the 

 plates of this Revision. A detailed account of the changes due to growth 

 will be found in the description of each species. 



In Cidaris (Pis. I, II, II C .) the difference between old (PL I. ; PI. II. 

 f. 1-S) and .young stages (PI. IP. f. ;, i.;) is almost entirely limited to the 

 proportionally larger size of the spines, and the more prominent serrations 

 l recalling Salenia). The abactinal system early assumes the character of the 

 adult ; in fact, with the exception of the smaller number of coronal plates, 

 the above differences in the spines are the only important changes under- 

 gone in this genus. The same holds good for Diadema (PI. IV. f. 1) and 

 Echinothrix (PL III", f. 1 -.;); in both genera the spines are proportionally 

 larger and less numerous : this gives to young Diadematidae a peculiar 

 facies, Echinothrix calamaria-like (PI. IP. f. e). We find also in young Dia- 

 dema characters in the actinal membrane differing from those of the adult; 

 the peculiar grouping, in five separate clusters, of the buccal ambulacral 

 plates which appear first, is soon lost by the encroachment of the smaller 

 interambulacral plates, and in older specimens the plates become deeply 

 imbedded in the buccal membrane. This feature is always retained in the 

 genus Centrostephanus. The pores at first are placed in a vertical row in 

 very young specimens (PL VI f. u a ) ; they then become arranged in arcs 

 of three (PL VI f. 15) or four pairs; with increasing age the median rows 

 of interambulacral tubercles assume the arrangement existing in the adult 

 (PL VI a .f. 5). Owing to the rapid growth of the spines in the young, the 

 extremity, and frequently the greater part of the spine almost to the base, 

 is hollow ; but as the young increase in age they become more solid at the 

 base, and farther up in proportion to their age (PL XXXV. f. 10). 



In the Cidaridae and Diadematidae the structure of the spines forms a good 

 basis for the discrimination of groups, notwithstanding their apparent great 

 changes of form. The variations do not extend to the nature of the ornamenta- 

 tion, which remains very constant, and will prove of great value in fossil Echini. 



