132 BULLETIN OF THE 



It is at about this time, PL V. fig. 12, ia 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, air, 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 anterolateral arise from separated lateral centres, and are latticed, 

 PI. VI. fig. 2, air. While the antero-internal, PL VI. fig. 4, air, origi- 

 nate on a medial line forming at a single centre. The rods of the two 

 arms, air, 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 extremity 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 



