290 DR J. STEPHENSON ON 
segmental arrangement, being present for some distance in front of each septum (fig. 5). 
In this situation, moreover, the intestine and dorsal vessel are above, the nerve cord and 
ventral vessel below the band. 
Other muscular strands are also to be seen (cf: fig. 5): thus fibres may be observed 
passing from the transverse bands in an oblique direction downwards to the body-wall. 
There are, in addition, a number of strands in the upper part of the body-cavity which 
pass between the intestine and the parietes ; these appear to be each constituted by a 
single cell, with a large ovoid nucleus situated to one side of the strand. 
In the most posterior part of the body the ventral portion of the ccelom may be 
completely filled by the blood-vessels, nerve cord, and a number of cells and fibres, so 
that there is here no free space below the level of the transverse bands (fig. 5). 
Circulatory system.—The dorsal vessel is, as described by Bepparp, not dorsal at — 
all for the greater part of its course ; it is situated ventrally, or rather ventro-laterally 
on the left side (figs. 3, 8), or it may be in places directly on the left of the 
intestine (fig. 5). Though not lying immediately on the intestinal wall, it has 
nevertheless a closer relation to the gut and to the chloragogen cells than has the 
ventral vessel, and the branches which unite it with the plexus on the intestine are much 
shorter than those which connect the plexus with the ventral vessel (fig. 8). In sections 
it is as a rule larger than the ventral vessel. It passes up the left side of the 
alimentary canal in the eleventh and tenth segments, and thenceforward is dorsal in 
position. In life it is contractile. 
The ventral vessel (figs. 3, 5) is on the right of the dorsal in the posterior part of 
the body ; it lies on or near the nerve cord; it is connected by numerous vessels with 
the intestinal plexus. Anteriorly it is formed by the union of the hearts. Its place 
is taken in front of the ninth segment by a trunk which is formed in segment vy by 
the union of two vessels coming from the anterior end of the body, and presumably 
originating in the forking of the front end of the dorsal vessel; this ‘anterior ventral’ 
vessel receives the lateral loop vessels on each side, including the large loops in 
segment vill from the supra-intestinal ; very soon thereafter it becomes enveloped in 
the chloragogen cells and disappears on the ventral surface of the alimentary canal. 
There is a rich intestinal plexus in the gut-wall, which in segments x—xiil becomes 
almost a sinus; sections through this region frequently show a continuous blood-space 
all round the circumference of the alimentary tube. Besides the connections with — 
dorsal and ventral vessels, the plexus is joined to the supra-intestinal by a series of | 
wide communications. . 
Owing to the comparatively large size and consequent opacity of my specimens 
during life, it was not easy to trace out in the living worms all the details of the course 
of the vessels to the gills and body-wall. Fig. 7 represents what could be seen; and 
in addition I have worked out the point in sections (fig. 8). In the gill-region the 
dorsal vessel gives in each segment two branches, one dorsalwards, on the inner surface 
of the body-wall (d.d.), which gives a twig across the mid-dorsal line and then enters 
