482 HIROSHI OHSHIMA 
to support arms. The dorsal arch consists only of the three- 
rayed portion, from which any vertical process fails to 
develop. The pre-oral and antero-dorsal rods also belong, 
according to this interpretation, to the simple type of the 
rods. In a similar manner, it seems to me, in Arbacia, 
Dorocidaris, HEchinocyamus, é&e., the posterior 
unpaired star fails to produce vertical processes, which would 
give rise to the aboral spike in the Spatangoid larva, the 
laterally directed basal arms being only developed as the 
postero-lateral rods. Prouho’s discovery of an abnormal 
larva of Dorocidaris papillata which produced a 
well-developed aboral spike (10, pp. 349-50, Pl. xxv, fig. 9; 
ef. Mortensen, 5, p. 75) is exceedingly interesting in this 
respect. Mortensen (5, p. 71) maintains that the state- 
ments of some authors, eg. Kolliker’s, who have 
described plutei with six to ten latticed rods must be wrong. 
My observations confirm this conclusion. Though in some 
abnormal cases those morphologically simple rods may be 
doubled or split, analogous to those I have observed, e.g. the 
dorsal horizontal rod of the right side (PI. 21, figs. 7 and 8, dh) 
and the left recurrent rod (fig. 6, re), it is quite impossible that 
they should assume latticed structure. 
Late in the gastrula stage a pair of calcification centres 
appear, which are bilaterally symmetrical in position. ‘This 
state of affairs is so well known in other Kehinoids that any 
detailed deseription is quite unnecessary. I may, however, 
pot out that the body-rod represents one of the three- 
rayed basal arms, not simply a posterior continuation of the 
postoral rod, as might easily be wrongly inferred because they 
both run in an almost straight line (fig. 1, br, po). ‘The 
other two arms of the base are represented respectively by 
the ventral horizontal rod (vh) and the recurrent rod (re), 
from which latter the antero-lateral rod (al) is given out later. 
The third, unpaired calcification centre appears near the 
posterior end of the body (ab). This may appear as early as 
in the stage where the future postoral arms can as yet hardly 
be recognized as arms, viz. when the larva has formed a slight 
