7o 



of the muscles. In other words, the "ectodermal pit" is simply a temporary crumple of the 

 skin, without any constant relation to the proboscis-pores. Thus fig. 42 shews no definite trace 

 of the structure, while in figs. 149, 150 (PI. XII) it appears to be well developed. But in 

 fig. 151, from the same series, the outer end of the groove is seen opposite the notochord 

 (nc/i.), while the proboscis-pore (/./. /.) opens at some distance in front of the groove; and 

 the same relations are already indicated in fig. 150. 



The proboscis-pores (PI. X, fig. 112'; PI. XI, figs. 137, 138; PI. XII, fig. 158; /./.) traverse 

 the central nervous system at a level which is indicated by the position of the pericardium and 

 the anterior dorsal horns of the collar-cavity. I have not certainly found them in the males of 

 C. sibogac, but their number and position are, e.xcept for this doubtful case, invariable. 



The strong nerves to the arms are direct lateral continuations of the central nervous 

 system. They are seen in the sagittal sections represented in figs. 149 — 141 (PI. XII), and in 

 the frontal sections shewn in figs. 113 — 118 (PI. X). The arm-nerve lies at the base of the 

 dorsal epidermis, which is often triangular in transverse section (figs. 141, 142). 



The nerve-layer at the base of the glandular epithelium of the proboscis is shewni in 

 PI. IV, figs. 36, 42, and elsewhere. 



The part of the central nervous system which extends on to the dorsal side of the 

 proboscis is connected with nerve-tracts running in a ventral direction along the posterior wall 

 of that organ. In C. dodecalophus I have been able to recognise two lateral tracts and a median 

 tract (PI. XII, figs. 152 — 155) which appear to correspond with Masterman's "pre-oral nerves". 



The central nervous system as seen in a median sagittal section (PI. IV, figs. 34, 42) 

 comes to an abrupt termination posteriorly at a point which corresponds with the hind edge of 

 the collar. On either side of the middle line it passes into the strong "lateral nerves" described 

 by Masterman (97, 2, p. 342; 98, 2, p. 513), with whom I do not, however, altogether agree 

 as to their further course, since I have been unable to recognise the independence of the two 

 nerves which in the diagram given by Masterman in his later paper (98, PI. V, fig. 85) are 

 supposed to run, on each side of the body, from the posterior end of the nervous system to 

 the stalk. According to my own results, there is on each side a single lateral nerve, which is an 

 extremely conspicuous structure passing from the central nervous system in the lateral ectoderm. 



In the first part of its course the lateral nerve is in contact with the basement-membrane 

 belonging both to the collar-cavity and to the metasomatic cavity. This is .seen in PI. XII, 

 fig. 150 (/.«.); while in fig. 151, a section of the same individual .slightly nearer the middle 

 line, the lateral nerve is separated by parts of the pharj-nx and body-cavity from the central 

 nervous system, still retaining its position adjacent to both the second and the third body-cavity. 



F"ig. 152 is a frontal section which passes somewhat ventrally to the central nervous system, 

 and therefore cuts the lateral nerves near their origin. The sub.sequent sections (figs. 153 — 155) 

 shew that the nerves pass behind the collar-canals, with which they are in contact. On reaching 

 the level of the gill-slits (fig. 156), behind which they lie, they are found to have lost all 



SIBOGA-EXriCDITIK. XXVI /'/V. lO 



