PHORMOSOM \ PLACES l L 33 



The characteristic pedicellariae of the genus appear early. A few pedi- 



cellariaa <»!' the different kinds noticed in the adull of this s] ies air found 



in young specimens scattered over the test, mainly on the actinal surface. 

 Sphaeridia are also presenl in these youngesl stages; they are round extend 



in-- from the plates of the actinal membrane, close i () the teeth, to the 

 abactinal area, along a line at the base of the ambulacra! tentacles, usually 

 one fur each tentacle ; in older stages they are rarely -ecu. being probably 

 broken off. If the Cidaridse possess sphaeridia, we may perhaps look for 

 them, in the young stages, on the imbricating plate- of tin.' actinal mem- 

 brane. 



In the early stages of growth tin 1 plates of the genital ring are in contact 

 along their edges; as the young become older, the space between the ocular 

 ami genital plates increases, ami ihe\ become separated by a number of anal 

 plate-. The anal system i- at first, in the youngesl specimens, covered by 

 plates of a nearly uniform size, with onh a few smaller ones occasionally 

 intercalated between them; with increasing size the number of these inter- 

 calated plates becomes larger, and the original larger anal plates an- then 

 separated by a greater number of accessory ones. The large original plates 

 retain their prominence In later stages of growth, and, much as the single 

 embryonic anal plate of young Echinoids, can easily he traced, in older 

 stages, among the other anal plates of subsequent growth. 



I do not quite understand Xeimiayr's statement that in the young Gly- 

 phostomes the anal plate is firsl formed, and that the plates of tie- genital 



ring are formed later and become detached from it laterally. That certainh 



is not the case in any of the young Echini I have had occcasion to examine. 

 While undoubtedly the anal plate is the first plate to appeal-, yet the genital 

 and ocular plates are formed outside ami independently "fit. just as much 

 as in the young stages of Comatula the basalia arise independently of the 

 dorso-central plate, and just as independently as the same plates arise inde- 

 pendently in the young Starfishes. See my Embryology of the Starfishes, 

 and Loven's figures. 



A- I bave previously shown in speaking of the apical system of the 

 Palaechinidaa and Echinothuriae, the plates of the apex of the anal system 

 hold a very differenl relation to the interambulacral system in those groups 

 from what they do iii the Echinidaa, in which the genital ring is closed, a 

 condition of things which lie-ins only with the Echini of Mesozoic time-, 

 and is represented in the Cidaridaa by their having -till a number of plates, 



