rnORMOSOMA PLACENTA. 33 
The characteristic pedicellarijB of the genus appear early. A few pedi- 
cellarife of the different kinds noticed in the adult of this species are found 
in young specimens scattered over the test, mainly on the actinal surface. 
Spha^ridia are also present in these youngest stages ; they are found extend- 
ing from the plates of the actinal membrane, close to the teeth, to the 
abactinal area, along a line at the base of the ambulacra! tentacles, usually 
one for each tentacle ; in older stages they are rarely seen, being probably 
broken off. If the Gidaridaa possess sphseridia, we may perhaps look for 
them, in the young stages, on the imbricating plates of the actinal mem- 
brane. 
In the early stages of growth the plates of the genital ring are in contact 
along their edges; as the young become older, the space between the ocular 
and genital plates increases, and they become separated by a number of anal 
plates. The anal system is at first, in the youngest specimens, covered by 
plates of a nearly uniform size, with only 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 are 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 be traced, in older 
stages, among the other anal plates of subsequent growth. 
I do not quite understiind Neumayr's statement that in the young Gly- 
phostomes the anal plate is first formed, and that the plates of the genital 
ring are formed later and become detached from it laterally. That certainly 
is not the case in any of the young Echini I have h.ad occcasion to examine. 
Wiiile undoubtedly the anal plate is the first plate to appear, yet the genital 
and ocular plates are formed outside and independently of it, 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. 
As I have previously shown in speaking of the apical system of the 
Palajchinidae and Echinothurise, the plates of the apex of the anal system 
hold a very different relation to the interambulacral system in those groups 
from what they do in the Echinidte, in which the genital I'ing is closed, a 
condition of things which begins only with the Echini of Mesozoic times, 
and is represented in the Cidaridoe by their having still a number of plates, 
