LARVAL SKELETON OF SPATANGUS 489 
stomach, but has gradually been pushed posteriorly, and the 
angles between the body- and recurrent rods of one side and 
their fellows of the other side are much widened. 
The twelve-armed specimen (figs. 7 and 8) is much younger 
than the larva figured by Mortensen (6, fig. 14), the total 
length measuring only 2-1mm. The pre-oral and postero- 
lateral arms are nearly equal in length, measuring 0-3 mm., 
a little shorter than the antero-lateral, which measure 0-35 mni. 
The antero-dorsal arms, which have appeared last, are only in 
the form of buds. The other arms and process are remarkably 
long, i.e. the posterior arms measuring 1 mm. in length, the 
posterior process 0-9 mm., and the postero-dorsal arms 0-8 mm. 
A short distance anterior to the poimt where the antero- 
dorsal rod is sent out from the dorsal arch, the latter produces 
a short lateral branch. The same is noticed by Miiller 
in Eechinopluteus fusus (9, Pl. vu, fig. 3) and by 
Mortensen in Echinocardium cordatum (5, p. 108, 
Pl. ix, fig. 6). In a Spatangoid larva, which has been doubt- 
fully identified by Mortensen (5, pp. 102-8) with Echino- 
cardium cordatum, Miller described and _ figured 
a peculiar feature in that the median posterior branch of the 
dorsal arch fused with the tips of the dorsal horizontal rods 
(8, p. 290, Pl. i, figs. 1 and 4, d). So far as I know such a case 
has never since been recorded by any other observers nor 
have I noticed it in my specimens (figs. 7 and 8, da, dh). 
The postero-lateral rod has no noticeable characteristics, 
being of a uniform thickness throughout and rather smooth, 
differing from the richly-serrated state as seen in Echino- 
cardium cordatum (Mortensen, 5, p. 103, Pl. ix, 
figs. 7and 8; MacBride, 4, Pl. xxxiu, fig. 11, pla). 
The rectangle formed by the body- and recurrent reds as 
seen In some younger stages (figs. 4 and 6) can no more be 
found (fig. 8). The area roughly corresponding to the anterior 
half of the rectangle is now occupied by an .irregularly-per- 
forated calcareous plate, which is developed more strongly 
on the right side than on the left side. The bases of the post- 
oral (po) and antero-lateral rods (al) are incorporated into this 
