47 



with regard to the structure of their collar-canals. The main relations of these organs seem to 

 be quite invariable. The problematical tissue is more highly developed in C. levinseni than in 

 C. dodecalopJuts. It is strongly developed in the neuter individuals of C. sibogae, in correlation 

 with the muscular nature of the animal generally; and it is less conspicuous in C. gracilis. 



IX. METASOME\ BODY or TRUNK. 



This region, which is characterized by including the third body-cavities, differs in proportions 

 in the different species, though there can be little doubt that the examination of contracted 

 preserved specimens does not give a complete idea of its shape when the strong antero-ventral 

 muscles are relaxed. The body is relatively short in C. dodecalophus^ while the opposite extreme 

 is reached in old individuals of C. levinseni^ in which the form is elongated and cylindrical. 

 C. gracilis and C. sibogae are intermediate between the other two species, in this respect. 



The metasome contains the whole of the alimentary canal, with the exception of the 

 anterior part of the pharynx, and it also includes the reproductive organs. Its junction with the 

 collar is not well marked externally on its anterior side, although it is sharply marked internally 

 b\- the septum between the second and third body-cavities, while the gill-slits open along the 

 line of junction. 



The alimentary canal and reproductive organs will be described below, and the present 

 Section deals principally with the third body-cavity. The main part of this cavity is a well 

 marked space, which is not obscured by connective tissue or muscles passing across it, as is the 

 case in the other coelomic spaces. It is subdivided by a median dorsal and ventral mesentery, 

 both of which are typically complete except for the fact that the ventral mesentery breaks 

 down in the middle of the stalk. This region is, however, traversed, on its anterior and posterior 

 sides by ridges (PL XI, figs. 132, 133) which indicate remains of the mesentery. The cavity' 

 of the stalk is usually filled by connective tissue, which ends sharply at the point where the 

 stalk joins the body (PI. IV, fig. 42). As the part of the ventral mesentery inside the body is 

 attached along its whole length to the surface of this mass of connective tissue, there is no 

 open communication between the two halves of the third body-cavity. The cavity is lined by a 

 peritoneum, which invests the surface of the ovaries and the whole of the alimentary canal, but 

 it does not pass into the region where the anterior wall of the intestine is in contact with the 

 posterior surface of the stomach. Included in the principal bend of the alimentary canal, in the 

 region of the second stomach there is, however, a small part of the body-cavity, which opens 

 widely, on either side, into the main body-cavity. This can always be seen in sagittal sections, 

 and it is marked b.c'^b. in PI. IV, figs. 33, 34, 37, I'i. 4^- It is indicated by M'Intosii (87) 

 in his fig. 3 on PI. Ill, and is alluded to as a "peculiar fold" in the wall of the alimentary 

 canal [t.cit., pp. 17, 41 note). The individual drawn in fig. 42 shews the arrangement described 



I) The stalk is of course to be regarded as part of the metasome, but it may conveniently be considered in a separate Section. 



