25 



The apparent oblieiuity of ihc body-cavities in young buds (PI. XIII, fig. 174) becomes 

 intelligible if it is assumed that the antero-posterior axis cuts the first and second body-cavities 

 at right angles. 



The dorsal region of the collar is distinguished by the development of the central 

 nervous system (hg. 42, e. 71. s.) which also e.Ktends on to the adjacent parts of the proboscis; 

 and of the paired tentaculiferous arms on either side of the middie line. Ventrally the collar 

 gives rise to the enormous lower lip or operculum (PI. III, fig. 25, o/>.), a structure which 

 must be of great importance in feeding, and in regulating the action of the gill-slits and collar- 

 canals. The coUar-cavities, as seen in a nearly sagittal section (fig. 42) are much larger dorsally 

 than ventrally, since the)- have to give origin to the cavities of the arms. A section which 

 cuts the whole of the main part of one of the collar-cavities thus has the form seen in PI. IV, 

 fig. 36 or PI. XII, fig. 151. 



The long axis of the metasome is at right angles to the antero-posterior axis. In 

 describing the parts of the metasome and stalk I shall consider the stalk as lying in the 

 position in which it occurs in buds and in the adults of most of the species; that is, in the 

 position shewn in PI. I, figs. 4, 6, 7. The pharyngeal side of the metasome, or that from 

 which the stalk originates, is the "anterior" side; while the rectal side is the "posterior" side. 

 The most "ventral" part of the body is, for descriptive purposes, the end of the hernia-like 

 projection containing the bend of the alimentary canal, although morphologically it more nearly 

 corresponds with the base of the stalk. 



The mouth (fig. 25, m.) is an orifice to which the food is conducted through a passage 

 formed, on each side of the proboscis-stalk, by the posterior wall of the ventral half of the 

 proboscis and the anterior wall of the collar, constituted by the arm-base («. ó.) and the 

 operculum or lower lip (o/>.). It leads into a pharynx (fig. 42, />/i.) from which a single pair 

 of gill-slits open to the exterior, laterally, but somewhat on the anterior surface of the body. 

 The external orifice of the gill-slit (fig. 25, £■. s. r.) is immediately ventral, or morphologically 

 posterior, to that of the collar-canal (f. c. c.) of the same side of the body ; and both orifices 

 lie so close to the base of the operculum that it is only in favourable specimens that they 

 can be seen from the outside without being viewed through the ventral lobe of the proboscis, 

 or through the operculum, or through both these structures. 



The pharynx passes into the oesophagus (fig. 42, ocs.) and these two parts together form 

 the commencement of the anterior limb of the f-shaped canal. The opening of the oesophagus 

 into the stomach {stom.) is usually borne on a large papilla which projects into the lumen of 

 the stomach between the dorsal and ventral caeca of the stomach seen in the figure on p. 23. 

 The second stomach (fig. 42, sto?ii.~) opens from the anterior side of the stomach, in the 

 immediate neighbourhood of the base of the stalk {st.). It passes round the ventral border 

 of the stomach, where it becomes continuous with the intestine {int.); this passes, in close 

 contact with the posterior wall of the stomach, into the rectum (r.) the anterior wall of which 

 is exposed to the body-cavity. The anus (fig. 22, an.) is a transverse slit on the dorsal and 

 posterior side of the metasome. 



A great part of the interval between the pharynx and the rectum is occupied (in female 



SIBOGA-EXPEDIÏIE XXVI (5;V. 4 





1^1. 



