490 HIROSHI OHSHIMA 
caleareous plate, and the recurrent rod is now hardly distin- 
guishable. Although it is difficult to make out clearly, it seems 
highly probable that neither the bases of the postero-dorsal 
(pd) nor of the postero-lateral rods (pl!) are fused with that 
plate. Similar features in the formation of calcareous plates 
are frequently met with in other irregular sea-urchins, e.g. 
Echinopluteus fusus (Miller, 9, Pl. iv, fig. 7; Pl. vu, 
figs. 3 and 11), Arbacia pustulosa (Miller, 9, Pl. im, 
figs. 2 and 3), &¢. Whether these plates have anything to 
do with the definitive skeleton of the young sea-urchin is 
still an open question, though it seems probable that they are 
absorbed altogether at the time of metamorphosis. 
SUMMARY. 
1. The larva of Spatangus purpureus reaches its 
last stage, which is characterized by its possession of six 
pairs of arms, in the course of three weeks after fertilization. 
2. The paired arms develop in the following order: post- 
oral, antero-lateral, postero-dorsal, pre-oral, postero-lateral, 
and antero-dorsal. The posterior process appears about the 
same time as the antero-lateral arms become distinct. 
3. These six pairs of arms and the unpaired process are 
each supported by a calcareous rod. Of these calcareous rods 
one can distinguish two classes which differ morphologically 
from each other, viz. the simple and the composite. 
4. To the class of simple rods belong the antero-lateral, 
pre-oral, postero-lateral, and antero-dorsal rods. They are 
either direct prolongations or branches of the three arms 
produced from one of the calcification centres. They are 
originally horizontal (parallel to the surface of the body) 
in position, and are homologous with the body-, recurrent. 
and horizontal rods. 
5. The remaining rods, viz. the post-oral and postero-dorsal 
rods and the aboral spike (posterior rod) are composite. 
They are each composed of three parallel rods connected by 
transverse beams so as to give a latticed appearance. Hach 
