66 University of California Publications in Zoology [Vol. 13 



Eberlein (1895, p. 243) points out, the brittleness of the cuticle is 

 due to the presence of silicic acid, there is probably silicic acid present 

 in this skeletal structure, for of all the structures of the body this is 

 at once the most rigid and the most brittle. The appearance in 

 surface view of these areas has already been described. In cross- 

 sections each of these regions is somewhat elliptical, transversely 

 crossed by thin, paired laminae {sh. lam., pi. 4, figs. 3-5; see also 

 microphotographs, pi. 7, figs. 23-29), extending from the cuticle per- 

 pendicularly inward to the inner wall of the elliptical space. At their 

 inner and outer attachments the members of each pair of laminae are 

 in close juxtaposition, but in the middle they spread apart, leaving a 

 minute elliptical interval between the two laminae. The appearance 

 in cross-sections is as though the inner wall of the elliptical space were 

 held away from the outer wall by a row of spindles, the longest of 

 which is situated in the middle. These pairs of laminae extend ob- 

 liquely posteriorly, corresponding with, and, as a matter of fact, 

 causing the longitudinal ridges in the cuticle over the above-described 

 skeletal structures. The central skeletal laminae of the left skeletal 

 structure, passing from anterior to posterior, at first grow longer and 

 longer, pushing the central portion of the inner wall of the ellipse 

 farther away from the outer wall until what was an elliptical area 

 becomes triangular in outline with apex pointed toward the longi- 

 tudinal axis of the body (pi. 7, figs. 23-28). Just above the middle 

 of the body, however, the reverse takes place, the central laminae 

 grow gradually shorter and allow the inner wall of the ellipse to 

 approach the outer wall until at about the middle of the body the left 

 skeletal structure joins the ventral skeletal structure, which- at about 

 the same level joins the right skeletal structure, i.e., the three skeletal 

 structures merge into one. The laminae of the left skeletal structure 

 (pi. 7, fig. 29) are at this level much longer than those of the ventral 

 and right skeletal structures, but they now become rapidly shorter, 

 soon disappear altogether, and this elliptical space with its contained 

 skeletal structure is entirely obliterated (pi. 7, figs. 30-32). The 

 skeletal elliptical spaces of the ventral and right sides fuse immedi- 

 ately anterior to the middle of the body (pi. 7, fig. 28). The laminae 

 soon become shorter, are placed more closely together, and appear in 

 cross-sections as a row of little pillars which support the oesophagus 

 and keep it away from the right wall of the body {v. sk. lam. and 

 r. sk. lam., pi. 4, fig. 5, and pi. 7, fig. 29). These laminae also 

 grow narrower and narrower up to the point immediately anterior 



