37 
radii, we can scarcely lay any stress on the case of the radially 
placed plates of the former. 
Among the Arbaciidæ two species, Arbacia punctulata and 
lixula have been studied as regards the development from the egg 
to the young Echinoid, the former by A. Agassiz, Garman & 
Colton and Fewkes, the latter by Joh. Muller (a. o.) It is, 
however, only Agassiz, who gives any indications of the devel¬ 
opment of the anal plates. In the „Revision of Echini“ (p. 733) 
Agassiz States that he has „traced in the youngest Arbacia raised 
from the pluteus the first appearance of the anal plates, which 
appear simultaneously as four lines radiating from the apex and 
forming the separations of the four anal plates u . This statement 
seems rather obscure — the plates appearing as lines separating 
these same plates —, and the figure 68 (p. 734) to which is 
referred, does not give any real solution of the question. In the 
„Panamic Deep Sea Echini“ (p. 54 — 55, PI. 53—54) some further 
notes and several admirable figures of the young, postlarval Arbacia 
are given, but the first formation of the anal plates is not shown. 
In the stage of 2 mm diameter these plates are evidently fully 
formed already, as appears from the figure 4, PI. 54, but in the 
younger stages Agassiz found it „difficult to decide if the anus 
is covered with four anal plates u . Garman & Colton 1 ) give a 
figure of a young, just metamorpbosed Arbacia, in which the four 
anal plates are distinctly seen (PI. XVIII, fig. 9); but the first 
appearance of these plates is not described by these authors either. 
The result is then that the first appearance of the anal plates has 
not been seen as yet. Another thing is that there can scarcely 
be any doubt that they really appear contemporaneously as four 
separate plates, and that a single large suranal plate is never 
found in the Arbaciids 2 ). There is nothing to support the suggestion 
9 Some notes on the development of Arbacia punctulata Lam. Studies 
Biol. Labor. Johns Hopkins Univ. II. 1882. 
2 ) H. L. Osborn has figured, in his paper «Variations in the apical 
plates of Arbacia punctulata from Wood’s Holl, Mass». (Science. N. S. 
