THE DEVELOPMENT OF ECtllNOCAKDIUM COKDATUM. 479 
firms (p.l.a., fig. 8), which arise from near the base of the 
aboral spike and which extend horizontally outwards. Each 
is supported by a p ostero -lateral rod (p.l.r., fig. 7), which 
is in reality only one of the lateral branches of the aboral 
calcareous star which has grown outwards. At the apex of the 
aboral spike there is a crest of ciliated epithelium [ah. cil., hg. 
8) which is entirely independent of the principal longitudinal 
ciliated band. By this time the hydrocoele has become marked 
out into incipient lobes which are the rudiments of the 
radial water-vascular canals and of the primary tube-feet of 
the adult [hy. L, fig. 8), and the amniotic invagination has 
become converted into a closed amniotic cavity [am., fig. 8). 
By the end of the second week a sixth and last pair of 
firms have made their appearance. These are the antero- 
dorsal arms; they spring from the dorsal surface of the 
oral lobe near its base and each is supported by an an ter o- 
d or sal rod which arises as a branch from the corresponding 
prte-oral rod. The larva has now attained its full complement 
of arms, viz. twelve, or if we count the aboral spike as an arm, 
thirteen, and for this reason Johannes Muller termed it the 
Pluteus with thirteen arms.’’ To judge from Morteusen’s 
(1913) figure it differs from the larva of Spat an g us of corre- 
sponding age in the fact that in the latter larva the aboral spike 
is as long as the rest of the body including the prse-oral arms, 
whereas in the larva of E chi no card iura, as fig. 9 shows, the 
aboral spike is only one fourth as long as body including 
prae-oral arms. The living larva at this stage is a marvel- 
lously beautiful object. All the thirteen firms are dotted 
over with patches of bright red pigment which is especially 
massed near their tips, and interspersed with the patches of 
this pigment are patches of a light yellowish-green pigment, 
and the two pigments combine to give the larva a wonderfull}^ 
gay appearance. 
The fully developed larva of Echinocardium has thus 
four larval arms not possessed by the larva of Echinus and 
in addition the aboral spike. Nevertheless it only possesses 
one more centre of calcification than is present in the larva of 
