42 



THE VOYAGE OF H.M.S. CHALLENGEE. 



of a rich brown colour owing to the existence of pigment in its walls ; the oviducts have 

 on previous occasions been described as eyes. 



Fig. 3 will serve to illustrate the anatomy of the collar-region of Cephalodiscus. 

 Whilst in the bud, this region is distinctly marked out by transverse grooves passing 



round the body of the animal, this distinct- 

 ness is no longer visible externally in the 

 adult form. The collar-region is, notwith- 

 standing, no less sharply marked out 

 internally than at younger stages. Its 

 body-cavity {b.c.-) is perfectly distinct from 

 the remaining coelomic spaces of the animal, 

 although it is to a considerable extent (and 

 especially in the lophophoral arms and the 

 operculum) filled by loose connective-tissue 

 (as in Balanoglossus). The dorsal part of 

 the collar is produced on each side into six 

 tentacular arms, into which (and into the 

 individual tentacles) the collar-cavity may 

 with ease be traced. The nervous system 

 {n.s.) extends on to the dorsal face of each 

 arm. The ventral border of the collar is 

 produced into a free fold, the operculum or 

 oral lamella, which reaches its highest deve- 

 lopment laterally and ventrally (with the 

 exception of the median line in the latter 

 region). It has before been stated that the 

 collar-cavities extend, dorsally, for some 

 distance along the proboscis region {vide fig. 

 2, h.c.^), and as a matter of fact, the anterior 

 limit of these cavities is coincident with the 

 origin of the most anteriorly placed lopho- 

 phoral arms. The collar is much less deve- 

 loped on the ventral side {h.c?, fig. 2), its 

 cavity in this region being, however, con- 

 tinuous at the sides of the alimentary canal with the dorsal part. The posterior border 

 of the collar passes on each side of the body along a line, which would be roughly 

 indicated in fig. 2 by joining the posterior end of the nervous system, dorsally, to the 

 base of the operculum, ventrally. This line corresponds, on each side of the body, with 

 the origin of the oral lamella or operculum. Since fig. 3 represents a section taken near 



Fio. 3. — Longitudinal right and left section tlirough an adult 

 Cephalodiscus, passing through the phamyx {ph.), ossophagus 

 ics.), stomach {st.), and intestine (int.) ; rus., nervous system ; 

 l.f lophophoral arms ; op., operculiim ; c.p., collar-pores ; gs., 

 gill-slits ; nies., dorsal mesentery ; b.c", body-cavity of collar ; 

 6.C.', body-cavity of trunk. 



