G6 SIR C. WYTirLE TIIOMSOX ON THE MODE OF 



These special spines are cylindrical and nearly smooth, and 

 they lean over towards the anal opening, and form an open tent 

 for the protection of the young, as in Cidaris nutria-, but at the 

 opposite pole of the body. In this species the eggs are extruded 

 directly into tlie marsupium ; and I imagine, from the very small 

 size of the ovarial openings, that when they enter it, they are 

 very minute, and probably unimpregnated. In the examples which 

 we dredged at the Ealkland Islands, the young were, in almost 

 every case, nearly ready to leave the marsupium ; we were too late 

 in the season to see the earlier stages. The young in the same mar- 

 supium are nearly ail of an age, some somewhat more advanced 

 than others. The diameter of the test is from 1 to l*o millim., 

 and the height about "S millim. ; the length of the primary spines 

 is, in the most backward of a brood, "5 millim., while in the most 

 advanced it equals the diameter of the test. The perisome, in 

 which the cribriform rudiments of the plates of the corona and 

 the young spines are being developed, is loaded with dark purple 

 pigment, which makes it difficult to observe the growth of the 

 calcareous elements. About thirty primary spines arise on the 

 surface of the corona almost simultaneously in ten rows of three 

 each : they first make their appearance as small papillse covered 

 with a densely pigmented ciliated membrane ; and when they 

 have once begun to lengthen, they run out very rapidly until they 

 bear to the young nearly the same proportions which the full- 

 grown spines bear to the mature corona. Yery shortly some of 

 the secondary spines, at first nearly as large as the sprouting pri- 

 mary spines, make their appearance in the interstices between 

 these ; and a crowd of very small spines rise on the nascent scales 

 of the peristome. Successively five or six pedicellarife nre deve- 

 loped towards the outer edge of tlie apical area, which at this stage 

 is disproportionately large ; the pedicellarias commence as purple 

 papillse, which are at first undistinguishable from young primary- 

 spines ; the first set look enormously large in proportion to the 

 other appendages of the perisome. Almost simultaneously with 

 the first appearance of the primary spines, ten tentacular feet 

 apparently the first pairs on each ambulacrum of the coronajust 

 beyond the edge of the peristome, come into play; they are very 

 delicate and extremely extensile, with well-defined sucking-disks ; 

 and Avitli these the young cling to and move over the spines of 

 the mother, and cling to the sides of the glass vessel, if thev 



