59 



open immediately behind an epithelial ridge (e. r.) which is visible in the former figures projecting 

 from the anterior side of the arm-base into the space between the proboscis and the collar. 

 The furrow on the median side of the ridge also opens into the mouth in the next section but 

 one. A similar arrangement is indicated on the right side, except that the posterior groove is 

 bifurcated at its internal end : the section being here at a slightly more dorsal level, the grooves 

 do not appear to open into the mouth. The posterior median groove of the diverticulum is 

 dying away, and the pleurochordal outgrowths are becoming larger. 



Fig. 156 cuts the mouth {tii.) and both the gill-slits {g.s.), the walls of the latter being 

 mainly composed of pleurochordal tissue; while fig. 157 shews that the pleurochordal tissue is 

 continued, in the walls of a pair of postero-lateral grooves of the pharynx, beyond the internal 

 openings of the gill-slits. There can be no question that, in this specimen, the posterior groove 

 of fig. 152 dies away between figs. 155 and 156, merging gradually into the surface of the 

 convex epithelium of the posterior wall of the pharynx; and that it cannot be traced beyond 

 the pleurochords, as indicated in Masterman's figs. 93 — 99. The median part of the operculum 

 in this individual is hanging ventrally from the mouth as an apron-like flap covering part of 

 the anterior surface of the body. 



Comparing these sections with the sagittal section, PI. IV, fig. 42, it appears that the 

 two grooves on each side of the middle line, opening into the mouth in fig. 155, pass horizontally 

 on one of the ventro-lateral sides of the upper lip to the ventral end of the anterior wall of 

 the pharyngeal diverticulum ; and that they here meet the two vertical anterior grooves of that 

 cavity. They are clearly the structures which Masterman (98, 2, p. 507) describes as the "oral 

 grooves". These are seen passing along the sides of the mouth in figs. 2 — 5 given by that 

 author, while fig. 6 cuts the anterior wall of the pharyngeal diverticulum (as is shewn by the 

 position of the notochord), the oral grooves opening into that structure on either side of the 

 median ridge, which is also shewn in my own fig. 152. Masterman's vfig. 7 accordingly passes 

 through the middle of the pharyngeal diverticulum. 



The oral grooves are well seen in a plasticine reconstruction (fig. 24) which I made 

 from a series of more or less transverse sections through an individual of C. levinsetii\ and in 

 this case three oral grooves are distinctly indicated on each side. 



Although I am thus able to confirm this part of Masterman's results I do not find 

 complete regularity in the arrangement of the oral grooves, which are not always symmetrical 

 on either side of the -middle line (fig. 155). A transverse section across the upper lip of C. 

 dodecalophus shews a consijicuous median ridge projecting into the mouth. This is derived in 

 front from the ventral wall of the proboscis, and further back from the collar, as will be obvious 

 by reference to fig. 42. On either side of the median ridge, there may be only a single lateral 

 ridge, so that two principal grooves pass along each side of the lip, as shewn in fig. 155. 

 The bifurcation of the lateral ridge may give rise to three grooves on one side. In fig. 24 

 (C. levinseni) there may be two ridges on each side, in addition to the median ridge which 

 projects from the proboscis-stalk; and three pairs of oral grooves are thus constituted. 



Even when I have found three oral grooves, on each side, I have not succeeded in 



