lOI 



coelom (cf. fig. 171) grows in between the stoniach and the inlestine froni either sicle until the 

 two layers come into apposition and tonn a meclian mesentery. The stomach and intestine are 

 separated, at the principal tlexure of the gut, l)y a section of body-cavity in the advanced bud 

 shewn in hg. iSi. It is perhaps significant tliat this is found at the end of that part of the 

 alimentarv canal which still retains, in this bud, what may be regarded as the primitive antero- 

 posterior direction. It is thus not impossible that the intervention of the body-cavity at this 

 point only of the flexure has some morphological importance in indicating the region at which 

 a dorsal flexure of the ahmentary canal was first initiated. 



I have but few observations on the later stages in the budding. In a bud in which 

 two pairs of arms have developed their terminal knobs, I find that the ventral mesentery has 

 already acquired its adult arrangement in the stalk, where its middle portion has broken down, 

 leaving merel)* the anterior and posterior mesenteric ridges which support the corresponding 

 stalk-vessels. Figs. 56 — 58 are sections transverse to the long axis of a bud of C. gracilis, 

 and shew (a) that the ventral mesentery is incomplete in the region of the stomach in the 

 buds of this species, as in the adult; {6) that the stomach and intestine are invested in a 

 common peritoneum, the intestine only acquiring a coelomic covering on its axial or dorsal 

 side when it becomes the rectum ; (r) that the dorsal vessel originates by two lateral roots 

 which are formed by folds of the splanchnic mesoderm covering the stomach, which is probably 

 surrounded bj- a system of vascular spaces between its own epithelium and that of the coelom. 

 This origin of the dorsal vessel is also indicated in the sagittal section of C. dodecalopims 

 shewn in fig. 181. 



Fig- 59 shews a section of an advanced bud of C. gracilis, cut transversely to its 

 long axis. The mesentery- of the third body-cavities is complete at this level, although it breaks 

 down in the middle immediately after the alimentary canal ceases to be visible in the sections. 

 The pharynx and stomach run in a horizontal plane, as in hg. 1 8 1 ; the next section nearer 

 the basal end of the stalk merely cutting the ventral wall of the stomach tangentially, while 

 the next section nearer the dorsal surface shews the intestine, which has started from the 

 posterior end of the stomach seen in fig. 59. 



The two collar-cavities (b. er) are seen at the sides of the mouth (;«.) the projecting 

 lobes of the body-wall in which they lie being parts of the operculum. Between the operculum 

 and the metasome, on the right side, is seen one of the collar-canals [c. c), which appear to be 

 formed, as Masterman states (98, 2, p. 518), as derivatives of the ectoderm. The evagination 

 ig. s.) of the pharj-ngeal wall seen on the right side can be identified as one of the gill-slits, 

 from its relation to the collar-canal; and here too I am confirming one of Masterman's results. 

 The proboscis in this specimen is not lying symmetrically. 



Fig. 5, of C. Icvinscni, is of interest as shewing that the proportions of the body, in 

 young individuals of that species, are not unlike those of the adult C. dodecalophus. 



