48 



by M'Intosh cjuite clearl\- in some of ihc more lulcral seclit^ns. The suggestion naturally occurs 

 that this arrangement may have some bearing on the manner in which the alimentary canal is 

 dirterentiated in the bud; a question which is considered in Section X\"l. 



The opening of this part of the cavity int(j the general metasomatic cavities is shewn 

 in PI. XI, fig. 130 and in PI. V, fig. 53. It is possible that the space is of functional importance 

 in allowing the second stomach to alter its diameter more freely than would be the case if its 

 movements were restricted by a close union of its wall with tliat of the principal stomach. 

 As this region of the mctasome is, moreover, probably exposed to considerable alterations in 

 shape or position, by the contraction of the antero-ventral musculature, it is possible that the 

 part of the body-cavity in cjuestion may have some importance in permitting a certain amount 

 of sliding motion of the two liml)s of the alimentary canal over one another, at the bend, thus 

 allowing the second stomach to adapt itself to alterations in the position or direction of the stalk. 



The peritoneal investment (//.), which is further considered below, is often so loosely 

 applied to the alimentary canal that a considerable space occurs between it and the digestive 

 e])ithelium (figs. 43, 51 — 53). This is clearly to some e.xtent the result of defective preservation, 

 but I believe that the investment is in reality a somewhat loose one, in certain places at least, 

 and that a splanchnic blood-sinus normally occurs below the peritoneum. 



The ventral part of the anterior wall of the third body-cavities is specially concerned 

 with th(> principal muscles, those which extend between se])tum ■/;, and the tip of the stalk. 

 A ventral horn {d. r.'' a.) extends, in the females, and in the neuter individuals of C. sibogae, 

 along the anterior border of the gill-slits (figs. 68, 77, 123, 156) as far as the region of the 

 collar-canals. These parts of the third bod\-cavity have already been discussed in the preceding 

 Section, and arc further described in Section XII, dealing with the musculature. 



Mesenteries. 



The mesenteries require sejjarate treatment for the difterent species. In C. dodecaloplius 

 and C. levinseiii the dorsal and ventral mesenteries are both complete, except in the axial region 

 of the stalk, where the ventral mesentery breaks down in all the species. 



In C. dodecalophus (PI. XII, figs. 152 — 157) the dorsal mesentery {mes'f) extends between 

 the dorsal body-wall, the pli.n \ nx, the rectum and the dorsal caecum of the stomach; another 

 detached portion of it bisecting the small cavity (PI. IV, fig. 42, d.c.^'d.) seen in sagittal sections 

 at the bend of the alimentar\- canal. The dorsal mesentery includes the well developed dorsal 

 vessel. I-'rom the dorsal mesentery are given off the lateral mesenteries, which carry blood- 

 vessels to the ovaries (PI. XII, figs. 152 — 157, ov.v.). 



The lateral or ovarian mesenteries {oz'. m.) extend from the dorsal body-wall and the 

 dorsal mesentery to the oviducts, terminating with a free edge ventrally slightly below the 

 point where the oviduct joins the ovary (PI. XII, figs. 149 — 156). Il is thus only the extreme 

 dor.sal end of the ovary which is su[)ported liy the lateral mesenter\ . In the region where the 

 oviduct opens to the exterior the lateral mesentery joins the dorsal body-wall (figs. 150 — 152). 



The ventral mesentery {v. /nes.'^) follows the convex side of the loop of the alimentary 

 canal. The rectum and intestine are so closely pressed against the posterior body-w^all that the 



