Gatty Marine Laboratory y St. Andrews. 35 



Protula, and anteriorly the ventral hypoderm is non-vascular. 

 Within is the circular coat which .extends all round, and 

 presents special developments at the foot. The dorsal 

 longitudinal muscles form massive kidney-shaped lobes in 

 transverse section, separated in the mid-dorsal line by the 

 alimentary canal and its short mesentery and by a vessel at 

 each side. These muscles extend from the dorsal almost 

 to the ventral edge, and are proportionally larger than in 

 •Protula. On the other hand, the ventral longitudinal are 

 smaller, and in section are short spindle-shaped bands widely 

 separated from each other, and with the nerve-cord and its 

 large neural canal at the inner edge. Between the latter 

 stretches a thin but continuous layer of longitudinal fibres, 

 having the circular muscular coat externally and the ventral 

 blood-vessel internally, with the muscular aponeurosis on 

 each side, as well as certain fibres from the slender oblique, 

 which passes the cord and is attached over the thin muscular 

 layer. The alimentary canal has a thick investment of 

 circular muscular fibres with groups of inner longitudinal 

 and a richly folded mucous lining. It stretches from the 

 dorsal surface to the ventral blood-vessel. The dorsal fold 

 arising from the foot is hollow distally. 



An interesting feature is the presence of a peri-intestinal 

 sinus in the outer wall of the alimentary canal and ex- 

 tending from the posterior region forward to the oesophagus, 

 and which takes the place of the dorsal vessel of other 

 forms, and the same arrangement occurs in the Ariciidse, 

 Chsetopteridaa, Ammocharidas, Sabellidse *, and other 

 families. 



The peri-intestinal sinus surrounds the canal throughout 

 the greater part of its extent, and in Eupomatus elegans 

 Prof. Haswell state's that the sinus ends in front of the oeso- 

 phageal region in a short wide dorsal sinus or cardiac sac, 

 from which a pair of vessels pass to each branchial base, 

 " where it (each) unites with a smaller branch from the ventral 

 vessel to form the common branchial vessel/' which makes 

 a curve — giving off a branch to each branchia and the 

 operculum and pseudo-operculum. "The ventral vessel is 

 a distinct wide trunk, which is continued along the body, 

 and in front communicates with the branches from the 

 dorsal sinus. The capillaries of the collar and flaps receive 

 blood from the ventral vessel, and, as in the branchiae, the 

 circulation is to-and-fro." The blood which enters the peri- 

 intestinal sinus by the segmental vessels is carried forward 



* Haswell, Proc. Linn. Soc. N.S.W. vol. ix. pp. 1-27 (sep, copy). 



3* 



