The Pluteus of Laganum sp. 131 
INTERNAL ANATOMY. 
THE LARVAL SKELETON. 
The larval skeleton is primarily a paired structure, each half of 
which supports one side of the pluteus. Each portion consists essen- 
tially of three parts: a transverse rod lying across the anterior end of 
the animal; its antero-posterior prolongation in the lateral wall of the 
pluteus; and a latticed rod supporting the corresponding larval arm. 
The transverse rods, after crossing the body in about the anterior 
third, bend nearly at right angles, and pass toward the posterior end 
of the pluteus. During their course they send out toward the periphery 
Fig. 2.—Dorsal portion of larval skeleton of Laganum, 55 hours, drawn from ventral side. XX 130. 
Fie. 3.—Ventral portion of skeleton of same specimen as fig. 2, also from ventral side. > 130. 
many irregular branches which anastomose repeatedly and gradually 
form a close network surrounding the animal on all but its ventral 
surface. The skeletal structure of this ventral region consists mainly 
of a branch from each primary lateral rod, which sweeps around ven- 
trally and may even cross that of the opposite side in the median line 
(fig. 3). These give out similar anastomosing branches, which, how- 
ever, never attain the high degree of complexity characteristic of the 
skeleton in the dorsal region. The network becomes finer just beneath 
the ectoderm, into which it projects innumerable short, spine-like 
processes. The skeletal supports of the arms are each made up of 3 
