90 SVEN LOVEN, OX THE KCHINOIDEA DESCRIBED BY LINN^US. 



ricliios, mnltiplying and radiatiug from the still erect protu- 

 bcrances, and now broken off över the sutures and beginnino- 

 to be cut iip, by contractions, into nodules, while the protu- 

 berances are approaclied by tlic rising tubercles, lowered and 

 caused to droop, sooii to be absorbed. In a specimen 9,5 mm. 

 in diameter, still devoid of sexual porcs, this juvenile aspect 

 is farther altered. On the calyx the pentagou is hardly dis- 

 cernible, the costals have löst their protuberances, the ridges, 

 still more increased in numbers have taken the form of beads, 

 numcrons contractions cutting them into very minute oval 

 nodules. The radials still have their three protuberances, 

 thongh somewhat reduced, or at least the middie one. On 

 the nppermost plates of the ambnlacra the epistroma keeps 

 up its carly form, two or three of its lirst protuberances stan- 

 diug upright, but on the third plate the tuberc vilar mamelons 

 appcar, pushing u]i the thiu envelope. On the interradia 

 the formation of primordial tubercles has already reached 

 the second plate from the top, and is fast reducing the lateral 

 protuberances, one or two of which only are left on the upper- 

 most plates, while on the disks they are seen on four plates, 

 but low and subsidiug, preparing to melt into other forms, 

 and around their bases the ridoes, nuraerous and bead-like, 

 assemble into a swelling layer. Lower down, on the fourth 

 or fifth plate from the top, the tiibercles of the second series 

 produce their mamelons, and upon their cones the glomeration 

 of the dwindling protuberance spreads, yielding and dissolving, 

 and ultimately leaves a trace only of its earlv presence, a 

 reddish iilm on their aboral half. 



When these juvenile features and the corresponding parts 

 in a fuUgrown specimen of the Arbacia fcquituberculata, Tab. 

 6\ Jig. 4, are compared together, the periproct and the anal 

 valves are seen to be much the same, but otherwise the dissi- 

 militude is very great. In the adult the costals and radials 

 are greatly changed. The same process that once had been 

 at work in remodelling the dorso-ccntral ossicle has extended 

 X<> them, the protuberances have long disappeared, and the 

 ridges, greatly increased in number, divaricating as from the 

 first, by a process indicated alreadv in the early stage and 

 gradually becoming more powerful, are cut up into vastly 

 multiplied croAvded nodules. On the interradia it is the same; 

 the berry-like bodics and connecting ridges, of which no trace 



