52 
might look like a central plate and five genital plates. This 
tigure, however, is so crude that it must seem quite unreasonable 
to conclude anything from it regarding the development of the 
apical system in Echinarachnius. 
The development of the plates covering the anal area is not 
described by Fewkes. But Agassiz has shown that the anal 
area is covered by a single large plate in the young Echinarachnius 
(Revision of Echini. PI. XII, fig. 11), so that there would appear 
to be a suranal plate developed here. 
The Me rido s ter nata do not show any trace of a central 
plate in tlie apical system, and likewise there is no trace of a 
larger suranal plate among the plates of the peri- 
proct. Any indication of the original condition 
being the existence of five radially placed anal 
plates is not seen either. It is true, that we 
know only the condition in the grown specimens. 
But there is no reason at all to suggest that the 
young postembryonal stages might possess a cen¬ 
tral or suranal plate. A young specimen of 
Urechinus naresianus of only 3 mm length („In¬ 
golf" Echinoidea II, p. 39—40) — the youngest 
specimen known of any of the Meridosternata — 
has its periproct covered with an outer circle of 
larger plates and some small inner ones (Fig. 12). There is 
nothing in this structure pointing towards the existence of pri- 
mordial suranal plate. 
In the Amphisternata, on the other hånd, a central 
plate is supposed by Loven to exist in those with an ethmolytic 
apical system, the great madreporic plate being regarded as homo- 
logous to the genital 2 -j- the central disk -j- the genital 5, 
which are all soldered together, the sutures having become lost. — 
(Etudes p. 83; Pourtalesia p. 71). It must be said that this inter¬ 
pretation of the large madreporic plate in the ethmolytic Amphi¬ 
sternata does not beforehand seem very probable. The mosl 
Fig. 12. Anal 
system of a 
young TJrechi- 
nus naresianus , 
of 3 mm length. 
8S /i. 
