OPHELIIILE. 3 



In this, as in the other forms, the dorsal and ventral vascnlar trunks and their branches 

 are conspicuous. A vessel leaves the latter on each side and passes to the branchia. 



The next stage in the transformation of the body-wall is seen in Ophelia (e.g. 0. 

 limadina), in which, while the oblique muscles are wide apart in front — with the nerve- 

 cords in the middle of the long interval between the ventral longitudinal muscles, the 

 cords having a granular area, a series of transverse fibres, the hypoderm, and the cuticle 

 internally — they wholly alter their position in the region behind. The oblique muscles, 1 

 in their progress backward, gradually increase in bulk, touch each other in the mid- 

 ventral line, push up the nerve-cords, and passing upward and outward as a wide 

 sheet, leave a broad flap beyond, viz., the ventral longitudinal muscle on each side 

 (Fig. 98). Soon, however, the muscle becomes shorter from side to side, but elongated 

 vertically so as to convert the lateral lamella into a pedicle with an enlarged end 

 containing the powerful ventral longitudinal muscle. The outline ventrally in section 



S.O.—P 



Fig. 97. — Transverse section of Ophelia A., dredged by the "Valorous/ d, alimentary canal; dm, dorsal longi- 

 tudinal muscle ; nc, nerve-cord ; om, oblique muscle ; pv, ccelomic cavity filled with male elements ; so, 

 segmental organ ; vm, ventral longitudinal muscle ; vv, ventral blood-vessel. 



thus shows an arch in the middle between two broadly clavate pillars, the inner ends of 

 the upper fibres of the oblique muscles touching in the middle line, the fibres being 

 attached to a pointed process of the cuticle which extends inward from the surface, and 

 having the nerve-cords above them. In this region the outline of the nerve-area is 

 ovoid, and more than one neural canal appears toward the centre. Externally is a 

 tough capsule and strands of neuroglia. The dorsal arch of the body is formed by the 

 dense cuticle, beneath which is a thin but firm layer apparently hypodermic, though 

 basement-tissue elements may also be incorporated internally. Within the foregoing is 

 the moderately developed dorsal longitudinal muscles, which fuse in the mid-dorsal line, 

 and pass downward on each side to the edge of the oblique muscle. The alimentary 

 canal in section anteriorly presents a tough homogeneous external layer with a 

 vacuolated external portion ventrally, a basement layer, and a thick internal coating of 

 columnar cells. In some the ccelom is distended with ova and in others with sperms. 



1 In the earlier paper in the ' Proc. Key. Soc. Edinb/ for 1876, these muscles were termed 

 vertical in Ophelia, but a more extended inquiry would seem to point to their being the oblique. 



