36 
the Echinothuridæ as in the Cidaridæ covered by five small anal 
plates u . In the young Phormosoma placenta the anal plates do 
not show any trace of 
five radially placed plates 
being the first to appear. 
As seen from the figure 
the outer circle oflarger 
plates shows a quite 
irregular arrangement, 
only one being radial 
in position. In the quoted 
place of the „Panamic 
Deep Sea Echini“ Agas- 
s i z refers to a figure 
(PI. 43, 2) of the apical 
Fig. 3. Apical system of a young Phormo- m Qf p hormosoma 
soma placenta , 3 mm in diameter. 28 li. 
placenta , in which „there 
seem to be five plates in [the angles of the anal pentagon some- 
what larger than the others“. It may perhaps be possible to 
point out in this figure five larger plates along the outer edge 
of the anal area, but these are placed off the genital plates, 
viz. interradially; only at one of the oculars a somewhat larger 
plate is seen. 
It may thus be stated that the little we know as yet of the 
development of the Echinothurids is decidedly not in favour of the 
supposition that either a central plate or five primordial radial 
plates are present in the young specimens. 
The embryological development of the apical system in the 
Diadematidæ is quite unknown. In the structure of the anal 
system of the grown specimens there is nothing pointing towards 
the existence of a primordial central plate. In Aspidodiadema 
tonsum we find five radially placed large anal plates; but since in 
another species of that genus (A. Jacobyi) there are 8 such cir- 
cumanal plates, placed, of course, without relation to radii or inter- 
