Oatty Marine Laboratory, St. Andrews. 29 



over the dorsum of both. The median band is somewhat 

 triangular with the pointed end internally, the outer is ovoid, 

 and in the hiatus between the muscles of opposite sides the 

 alimentary canal is suspended, and so closely that no 

 mesentery is apparent — indeed, it would seem that the mus- 

 cular fibres which pass from the circular coat into its walls 

 form the suspensory apparatus. Ventrally the longitudinal 

 muscles likewise form in transverse section two areas, in 

 this case somewhat heart-shaped, the base of each being- 

 central, the apex external, and the outer (ventral) fillet of 

 the muscle likewise extends over both areas. The inner 

 edge of each muscle is separated by a considerable gap, in 

 which lie the nerve-trunks which rest in a granular neuro- 

 glia, with the neurilemma and the circular muscular coat 

 externally, whilst to their upper edge are attached strands 

 from the alimentary canal. The two cords are surrounded 

 by a sheath or neurilemma, and at the upper and inner 

 angle is a small neural canal. At the ganglia the neuri- 

 lemma is confined to the outer surface. In the mid-ventral 

 line beneath them is a granular mass (in section) of neuro- 

 glia, and a trace also appears at each side, whilst in the 

 region of the separate cords this inferior granular structure 

 is thicker in the centre 'and tapers off laterally. On each 

 side of the strands from the alimentary canal is a foliate 

 granular mass (male elements ?), whilst between the strands 

 is the ventral blood-vessel. Large vascular trunks or 

 sinuses occur along the wall of the alimentary canal. The 

 fan-like arrangement of the long hooks is well shown in 

 such sections. 



Somewhat behind the foregoing the mid-dorsal groove 

 becomes only a slight depression, though the hvpoderm 

 retains the same character as in front and the cuticular 

 surface appears to be ciliated. The hypoderm now forms a 

 coat of nearly equal depth all over, though there is still 

 a slight thickening in the mid-ventral line due apparently 

 to increase in the basement-substance as well as in the 

 hypoderm proper. The circular coat has increased in 

 strength, the suspensory fibres for the alimentary canal are 

 longer, and the canal itself shows both circular and longi- 

 tudinal fibres, whilst the folds of the mucous surface are 

 sometimes so arranged in the empty organ as to interlock. 

 Strong fibres at intervals pass from the dorsal to the ventral 

 region — grasping the alimentary canal at each side, and 

 being attached to the fibres, including those of the oblique 

 muscles, which form a complex around the ventral blood- 

 vessel and over the nerve-cords. The latter have now, at 



