sections drawn (PI. V, figs. 43 — 53) begin with one which passes nearly medianly through the 

 phar)-n.\ (tig. 43) and end with a section (fig. 53) through a plane in which all the five arms 

 have separated fi-oni the arm-base. Two points must be noticed in interpreting these sections : — 



(I) the intestine appears to open into the middle of the posterior side of the stomach (figs. 43, 44). 

 I find no trace of this arrangement (which has not been shewn in fig. 22) in other specimens 

 of C. gracilis^ and I have no doubt that it is due to an artificial rupture or to an abnormality : — 



(II) Owing to a strong lateral curvature of the body (which has not been brought out in fig. 22), 

 the end of the metasome connected with the stalk becomes separated, in the sections, from 

 the anterior end. 



Fig. 43 represents a section, which passes somewhat to the left of the ]jroboscis-stalk. 

 The proboscis (/.) is thus completely separated from the collar. The operculum (<?/.) in this 

 individual is directed forwards, and has the appearance of a lower lip. The dorsal part of the 

 collar is produced in front into the left arm-base, with whicli the third (/,. 3) and fourth [L. 4) 

 arms are connected '). The collar-cavity [b. c.~) extends as a wide space into the arm-ba.se, and 

 is separated by an oblique septum from the third body-cavity {d. c:'). On the front side of the 

 collar-cavity is the oral muscle (07'. in.), a strong band of longitudinal muscle-fibres on the 

 inner side of the anterior bodx-wall. The collar-cavity is interrupted by the mouth {in.), on the 

 ventral side of which is the part of the cavity which e.xtends into the operculum. 



Fig. 44 represents a section which passes nearly through the middle of the proboscis- 

 stalk. The fu-st and second left arms are now continuous with the arm-base. The septum 

 between the first and second body-cavities ■) contains the notochord {iich.), starting from the 

 anterior side of the median dorsal diverticulum {div!) of the pharynx, which extends completely 

 up to the dorsal body- wall. The posterior wall of the dorsal part of the cavity contains part 

 of the central nervous sy.stem [c. n. .$■.). In other respects the collar in this section resembles 

 what was described in the previous section, except that it is now continuous with the proboscis. 



In \\o, 45, the middle line has been passed, so the collar-cavity is that of the right 

 side. The right edge of the mouth has just been passed, and a strong oral muscle [or. in.), 

 corresponding with the similar left muscle shewn in hg. 43. is seen to extend from the septum 

 ■/a round the edge of the mouth to the anterior wall of tiie arm-base^ The septum '/_• has a 

 crenulated appearance, and from it radiate numerous mu.scle-fibres which pass through d. f.' 

 to the anterior wall of the proboscis. The internal opening of the right proboscis-pore {p-p-) 

 is seen, and the whole extent of the central nervous system (r. n. s.). 



In lig. 46, the right edge of the proboscis-stalk is cut, and the collar-cavity (/'. c.-) is 

 continuous from the arm-base {a. d.), dorsally, round the right side of the mouth, to the 

 operculum, ventrally. The operculum is clearly a derivative of the anterior edge of the collar, 

 while the collar-canal [c. c.) occupies a ventro-lateral recess of the second body-cavity, immediately 

 in front of the gill-slit. As the pharynx no longer reaches the dorsal body-wall, the second 

 and third body-cavities are separated by a thin septum (s. 7,1). 



1) It will of course be noticed ihal the arms shewn in the reconstructions, figs. 22, 25, are those of the right side. 



2) It will be convenient to indicate this as "septum '/j", and that between the second and third cavities as "septum '/a''- 



