49 



mesentery cannol always be detected in this re<^ion (li^s. 152 — 157). In some cases, however, 

 the posterior limb of the alimentary canal is seen to be suspended by an extremely short 

 mesentery, the coelom thus intervening between the canal and the body-wall. In favourable 

 specimens (fig. 157) the anterior part of the ventral mesentery is seen to include a small blood- 

 vessel [a. z''.), wliich occurs at the junction of the mesentery with the body-wall. This vessel is 

 the direct continuation of the anterior vessel (fig. 133, a. v.) of the stalk. 



The mesenteries of the third body-cavity of C. levinseni agree in all respects with those 

 of C. dodccalophus, as may be seen from the frontal sections shewn in figs. 120 — 130. The 

 dorsal and lateral mesenteries are shewn in figs. 119 — 123. In figs. 124, 125, the lateral 

 mesenteries have come to an end, while the dorsal mesentery continues uninterruptedly until 

 the stomach is reached. Fig. 1 30 shews the detached part of the dorsal me.sentery which occurs 

 at the bend of the alimentary canal. The extreme anterior end of the ventral mesentery, with 

 its blood-vessel, is seen in hg. 124, between the pharynx and the ventral part of the collar. 

 The ventral mesentery can be seen, on the anterior and posterior sides of the alimentar)- canal, 

 in the remaining sections figured (figs. 125 — 130). 



In C. gracilis^ perhaps in correlation with its smaller size, the mesenteries are less 

 complete than in the two species so far considered. The dorsal mesentery is complete only at 

 its extreme dorsal end (PI. VI, fig. 67), while in sections nearer the stalk (figs. 68 — 70) it is 

 represented merel)' by the dorsal vessel. The lateral mesenteries are similarly represented by the 

 vessels to the ovaries (fig. 68). The ventral mesentery can be distinguished on the pharyngeal 

 side (fig. 69) for a short distance beyond the coUar-region, but it soon disappears completely 

 (fig. 70), except so far as it is represented by the blood-vessel, which in favourable specimens 

 can be seen on the inner side of the body-wall. The rectum is usually in such close contact 

 with the posterior body-wall that no definite mesentery is formed on this side. A portion of the 

 ventral mesenterj- persists near the base of the stalk, where it carries the posterior stalk- vessel 

 to the bend of the alimentary canal (fig. 71). I have not detected any portion of the dorsal 

 mesentery on the concave side of the bend, in the part of the body-cavity marked b.c?b. in 

 fig. 37; but I am not prepared to .say that it does not exist in that region. 



It may be noted that the ventral mesentery is incomplete even in the buds of C. gracilis 

 (PI. \\ fig. 56) whereas in C. dodecaloplms the observations of Masterman (98, 2, figs. 52 — 58, etc.) 

 shew that it is complete at a corresponding stage. 



In the neuter individuals of C. sibogae the dorsal mesentery is incomplete (PI. VIII, 

 figs. 90, 91). The lateral mesenteries are not present, in consequence of the absence of gonads. 

 The ventral mesentery is distinguishable, on the pharyngeal side, in the region of the gill-slits 

 (fig. 92). In the males of the same species there are remains of the donsal and lateral mesenteries 

 (figs. 86, 87) and of the ventral mesentery (fig. 89). 



The body-wall of the metasome is in most parts thin and without any .specially noteworthy 

 characters. The muscular layer is almost confined to the anterior surface (see .Section XII). 

 The epidermis may be pigmented, especially in C. sibogae, which is characterized by the presence, 

 in practically all parts of the adult animal, of an unusually large amount of very dark pigment. 



SIBOGA-EXI'EDITIE XXVI i^/i'. 7 



