Gatty Marine Laboratory, St. Andrews. 31 



hypoderni is greatly developed on the ventral surface, thin- 

 ning oft' in the lateral regions, and with a slight groove mid- 

 dorsally. The circular muscular coat is fairly developed 

 all round. The dorsal longitudinal muscles form a con- 

 tinuous loop in transverse section, the broader end of each 

 being external, and the short mesenterial attachment of the 

 alimentary canal separates each muscle in the mid-dorsal 

 line. The folds of the ventral longitudinal museles are also 

 apparently continuous in section, both these and the dorsal 

 being somewhat lappet-shaped, the inner end being pointed, 

 the internal fold of the muscles terminating before reaching 

 the point in each case. The oblique muscles seem to be 

 feeble and indistinct, each appearing as a thread-like process 

 along the inuer border of the ventral longitudinal muscle, 

 and being attached over each nerve-cord. The alimentary 

 canal (gullet) is large in this region, and has a firm exterior 

 with circular and longitudinal muscular fibres, and a thick 

 mucous coat, the nerve-cords are comparatively small and 

 lie in the intervals between the ganglia in the middle line 

 below the attachment of the mesentery from the gut. Ex- 

 ternally are a mass of neuroglia, the circular muscular coat, 

 and the much thickened hypoderm of the ventral surface, 

 which shows no median groove in this region. The nerve- 

 area is considerably larger when a ganglion is severed. The 

 canal is ensheathed by a firm mesentery fixed on each side over 

 the nerve-cords. A small canal occurs in the median line 

 above the nerve-cords, and the gonads are at each side. 

 The sheath of the alimentary canal is close to the vessel, 

 thus differing from the usual condition of a free space 

 between loose mesenteries. 



A little (£ in.) behind the foregoing the ventral surface is 

 marked by a deep groove, so that the thick hypoderm forms 

 a crescentic mass on each side. The alimentary canal is 

 much enlarged, and its lumen filled with folds of mucous 

 membrane. The dorsal and ventral longitudinal muscles 

 have the same structure in section. 



Toward the posterior region of the body, whilst at first 

 the ventral muscles indicate no change, the dorsal loop 

 presents a hiatus at the ventral edge on each side of the 

 middle line, from which apparently the homologues of the 

 oblique muscles pass, the outer fold being enlarged next 

 the fissure; such is the condition at ^ of an inch from the 

 tip of the tail. The whole aspect of each muscle, however, 

 alters at about \ of an inch from the tip of the tail. Each 

 dorsal muscle forms in section a continuous thick arch 

 superiorly, the inner end bending downward and forming a 



