34 DR J. STEPHENSON ON 
forms. In segments xiv.—xvill. doubled-pronged and palmate setee may be mixed ; or the 
bundle on one side may consist of double-pronged, on the other side of palmate setze. 
The circulatory system, briefly referred to by BENHAM, deserves description (PI. I. 
fig. 1). The dorsal vessel is connected with the alimentary wall and covered by 
chloragogen cells as far forwards as the tenth segment, where it becomes free ; from the 
seventh to the second segment it gives off prominent lateral loops, non-contractile, 
tortuous, running on the inner face of the body-wall. - The dorsal vessel bifurcates at 
the junction of prostomium and first segment; the branches unite again below about the 
level of the setee of segment iv. The ventrally situated vessel which is thus formed is 
outside the chloragogen cells ; 1t unites posteriorly with the subintestinal in the eighth 
segment. 
The supraintestinal vessel is present on the alimentary canal, covered by chloragogen 
cells, from about the place where the dorsal vessel leaves the intestine to the fifth 
seoment anteriorly. It gives origin in segment vill. to the two hearts, greatly dilated 
vessels, one on each side, which contract from above downwards, and, as BENHAM has 
ring 
Fie. 1.—a, a multidentate seta from the fourth segment of a specimen of Z'whifea costatus. 
b, intermediate forms of sete from the fourth segment of a specimen of Twbifex 
costatus . 
a 
remarked, alternately ; these hearts, approaching each other ventrally at the level of 
septum $, are prolonged almost parallel to each other, without immediately uniting, 
backwards through the ninth segment; they then join to form the ventral vessel, 
which is continued backwards below the alimentary tube in the body cavity. 
There remains to be mentioned the subintestinal vessel, a single median channel, on 
the intestinal wall within the investment of chloragogen cells. This can be distinguished 
in sections as far back as segment xiv. ; in seoment xii. it is equal to the ventral vessel 
in size, in segment x. larger; it soon becomes small again, and dies away on the intestine 
in the anterior part of the eighth segment, after receiving the posterior end of the 
ventrally situated vessel previously described. The relations of these several vessels are 
illustrated diagrammatically in Pl. I. fig. 1. 
The parietal plexus is most copious in the posterior part of the body; the loops 
branch and reunite on the inner surface of the body-wall, but do not penetrate the 
circular muscular coat; these branches on the body-wall are of considerable size— 
indeed, are of the full diameter of the loop which gives origin ‘to them before it 
divides up. 
The shape of the cerebral ganglion is sometimes made use of in specific diagnoses. 
