Gatty Marine Laboratory, St. Andrews. 53 



evidence of a typhlosole in this region occurs. Moreover, 

 whilst the ventral vessel remains in position, the vascular 

 branches on the walls are inconspicuous, though they seem 

 to form a reticulate series. This part of the gut is often 

 loaded with sandy debris, surrounded by the dilated but 

 tough investment of the gut-wall, which appears to contain 

 inner longitudinal and circular muscular fibres, though these 

 are only visible in some sections, the tough investment 

 in dilatation being apparently homogeneous, as observed in 

 cases where the cylindrical epithelium has disappeared by 

 maceration. 



Nervous System. — The cephalic ganglia occur behind the 

 bases of the branchiae, their anterior border appearing about 

 the level of the base of the opercular stalk as it begins to 

 project from the somewhat quadrangular outline of the body 

 in section. They form a fused mass above the oesophagus, 

 supported in front by a dense group of nucleated cells with 

 slight differentiations -at each side, probably indicating the 

 issue of nerves. Then a somewhat narrow band appears, 

 chiefly of transverse fibres with two large nerves passing off 

 at each end, one entering the base of the operculum on the 

 left and the other entering the lateral tissues, whilst those 

 on the right go to corresponding parts. The central part of 

 the ganglia behind increases in bulk, the organ forming a 

 broad band with an enlargement at each end, the whole 

 surrounded by a coating of the nucleated cells, and many 

 transverse commissural nerve-fibres appearing in the centre. 

 The outer enlargement then bends downward and elongates 

 ventrally, the transverse commissural fibres still persisting 

 between the sides, but finally these are gradually replaced 

 by the nucleated cells, and the great nerve-cords, widely 

 separated, lie on each side of the oesophagus. Before this 

 occurs, however, long commissural fibres pass between 

 the trunks over the oesophagus. There is thus a variation 

 from the ordinary arrangement in typical forms, in which 

 these cords slant below the oesophagus and meet more or 

 less closely in the first ganglion of the chain. The nerve- 

 cords are wide apart in the region of the muciparous 

 glands, and it is just after these have been passed in the 

 backward progress that a small neural canal is observed at 

 the inner end of each trunk — still at a considerable distance 

 from its fellow, and with the fibres of the special interneural 

 bands of longitudinal muscular fibres still present. The 

 nerve-trunks lie at the inner edge of each ventral longi- 

 tudinal muscle, which forms a comparatively thin plate on 



