132 BULLETIN OF THE 
It is at about this time, Pl. V. fig. 12, in the growth of the pluteus 
that we see signs of the formation of two additional pairs of rods, one 
of which is known as the antero-lateral, alr, the other the antero-inter- 
nal. These two pairs of rods have certain points in common, as far as 
their mode of formation goes. Neither of them arises from the primi- 
tive centres of calcification from which the rods already formed invari- 
ably spring, and to which they are fused. Of these two pairs of rods 
the antero-lateral arise from separated lateral centres, and are latticed, 
Pl. VI. fig. 2, alr. While the antero-internal, Pl. VI. fig. 4, aw, origi- 
nate on a medial line forming at a single centre. The rods of the two 
arms. a7, are simple, smooth, sometimes with small lateral spurs or 
teeth and not latticed. 
The antero-internal spicule or rod is well formed in the body of the 
pluteus before any projection on the surface at the point where it later 
appears can be noticed. It arises as a trifid spicule in the basal region 
of the oral lobe. As it grows it becomes crescentic, the convexity 
turned to the oral lobe, and the two horns extend about parallel with 
the anterior rods. A slight spur or median tooth arises from the con- 
vex side of the crescent. The first appearance of the antero-internal 
arms is marked on the surface of the larva by a projection on each side 
of the anterior lobe within the anterior rods. Into these projections, 
as they increase in size, the Gxurenuey of the two horns of the crescentic 
spicule extend. 
The antero-lateral rods, al, Pl. VI. figs. 3, 4, Pl. VIII. fig. 5, originate 
in a different way from the single median centre of calcification of the 
antero-internal. They arise, not from a single centre of calcification, but 
from two lateral centres. Just above the anterior lobe, in the interval 
between it and the base of the posterior arms, a projection forms on the 
edge of the plutean body. This projection raises with it a section of the 
circumoral ciliated band, and in its interior there forms a pair of rods 
which become joined and form a latticed rod, resembling the posterior 
rod. The antero-lateral rods are not fused with the other rods, and as 
by the growth of the antero-lateral rods little by little the arm equals 
in length that of the posterior, pari passu the rod grows without joining 
the remaining spicules. It is this freedom of the two systems which 
renders it possible for these arms to be moved by the muscles of the 
pluteus. The antero-internal system of rods also does not join the 
other rods, and is likewise movable, while the muscular fibers which 
accomplish this are easily seen near its junction with the anterior arms. 
The pluteus is now, Pl. VI. fig. 4, in about the same stage as that 
