48 



by M'Intosh quite clearly in some of the more lateral sections. The suggestion naturally occurs 

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

 differentiated in the bud; a question which is considered in Section XVI. 



The opening of this part of the cavity into 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 that of the principal stomach. 

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

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

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

 of sHding motion of the two limbs 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 (pt.)^ which is further considered below, is often so loosely 

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

 epithelium (figs. 43, 51 — 53). This is clearly to some e.xtent the result of clefective 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 thirci body-cavities is specially concerned 

 with the principal muscles, those which extend between septum -73 and the tip of the stalk. 

 A ventral horn {b.c?a^ extends, in the females, and in the neuter individuals of C. sióogae, 

 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 body-cavity have already been discussed in the preceding 

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



M e s e n t e r i e s. 



The mesenteries require separate treatment for the different species. In C. dodecalopJnis 

 and C. Icvmseni 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. dodecalopJitis (PI. XII, figs. 152 — 157) the dorsal mesentery {mes'/') extends between 

 the dorsal body-wall, the pharynx, the rectum and the dorsal caecum of the stomach; another 

 detached portion of it bisecting the small cavity (PI. IV, fig. 42, è. l\^ ó.) seen in sagittal sections 

 at the bend of the alimentary canal. The dorsal mesentery includes the well developed dorsal 

 vessel. F"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 [ov. m.) extend from the dorsal body-wall and the 

 dorsal me.sentery to the oviducts, terminating with a free edge ventrally slightly below the 

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

 dorsal end of the ovary which is supported by the lateral mesentery. 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. mes:'') follows the convex side of the loop of the alimentary 

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



