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 



Fio. 2. Dorsal portion of larval skeleton of Laganum, 55 hours, drawn from ventral side. X 130. 

 FIG. 3. Ventral portion of skeleton of same specimen as fig. 2, also from ventral side. X 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 



