484 HIROSHI OHSHIMA 
seen and lasts for a fairly long period, and it seems to me that 
this can be regarded as a specific character in identifying 
Spatangoid larvae. In many other Spatangoid larvae this 
is not the case ; these rods either stand apart or pass across, 
as in Hehinocardium cordatum and its doubtful ally 
(Miller, 8, p. 290, Pl. in, fig. 2). In cases where both ends 
come very close together, as in Echinopluteus fusus 
(Muller, 9,-Pl. vii, fig. 2), Heesolidup (9)Pl. vi, fie 95 
Pl. vu, fig. 1), and perhaps Brissopsis lyrifera also 
(Mortensen, 7, fig. 2), they do not form any thickened 
joint. Only in Chadwick’s figures of an unidentified 
form (1, Pl. ix, figs. 61 and 62) the similar state of the ventral 
horizontal rods is very clearly shown. 
By the time when the two-armed stage is fully developed, 
when the post-oral arms have reached the length nearly equal 
to the body proper, whereas neither the antero-lateral arms nor 
the aboral process are as yet distinct, the following features 
are to be noticed: the post-oral rods are usually solid and 
three-ridged, and the margin of the ridges is not serrated. 
lixceptionally, however, some irregularly-scattered holes 
may be met with even near the proximal end of the rod, but 
owing to the very slight differences in the refractive indices 
between the thin, filmy skeleton and the surrounding medium, 
which consists of oil of cloves or Canada balsam, it is diffieult 
to demonstrate the holes clearly. Krohn (2, p. 256) observed 
no fenestration in these rods in the corresponding stage. 
Further, in his figure (Pl. vu, fig. 1) he showed only the antero- 
lateral and body-rods besides the post-oral, whilst the ventral 
horizontal and recurrent rods are not represented. The star 
of the aboral spike should also have appeared in this stage. 
The recurrent rod grows rapidly, and when its posterior end 
comes in contact with that of its fellow of the other side 
(fig. 2, re) fuses with it and increases in thickness, often being 
beset with some irregular short processes near the end 
(fig. 8, re). A little anterior to this end a branch is soon sent 
out ventrally, while about the same time the body-rod 
produces a branch dorsally, and these two branches meet 
