INTESTINAL RESPIRATION IN ANNELIDS. 765 



in close connection with the alimentary tract, since these illustrate the increasing 

 differentiation of the vascular system, and will be again referred to in the next section. 

 I omit the ventral vessel, which is quite separate from the intestine, the parietal 

 and subneural vessels, the hearts, the anterior loops, and the nephridial plexuses. 



In Pheretima posthuma the dorsal vessel lies on the intestine, attached loosely to 

 it in each segment by two pairs of vessels to the intestinal v/all ; these vessels join with 

 corresponding branches from the supraintestinal and then pass transversely outwards 

 on the intestine. The dorsal vessel is not covered by chloragogen cells ; it becomes 

 quite free anteriorly about segment x., and runs forwards at some distance above the 

 alimentary canal, piercing the septa in succession, giving branches to the body-wall, and 

 ending in front by disappearing into the wall of the pharynx at the level of the 

 cerebral ganglion. 



The supraintestinal vessel is distinguishable anteriorly about segment x. as a mid- 

 dorsal channel in the alimentary wall ; it extends back along the oesophagus and 

 intestine beneath the dorsal vessel, communicating with the hearts in segments xii. 

 and xiii., and with the alimentary plexus throughout its extent. 



A subintestinal channel can be distinguished in the mid-ventral line on the anterior 

 part of the intestine ; it ends in front by dividing into a pair of ventro-lateral vessels 

 in segment xiv., and posteriorly becomes indistinguishable behind the level of the 

 intestinal caeca (segment xxvi.) ; it is a portion of the intestinal plexus, and hardly 

 deserves a separate name. 



The intestinal plexus consists of (1) transverse vessels, two pairs per segment, each 

 formed by the union of a branch from the dorsal with one from the supraintestinal 

 vessel; (2) close-set longitudinal channels, given off from the above transverse vessels, 

 connecting successive transverse vessels with each other, and forming the main portion 

 of the plexus ; (3) oblique vessels, which, beginning near the mid-ventral line, incline 

 forwards and dorsalwards, and pass through about three segments before reaching the 

 mid-dorsal line, where they join the supraintestinal ; (4) on the intestinal caeca are 

 a number of longitudinal channels, meeting at the tip of the cseca, and connected in 

 their course by numerous transverse vessels ; (5) along the lateral aspects of the anterior 

 portion of the intestine, from segments xiv.-xxvi., are a pair of well-marked vessels, 

 which, beginning ventrally in the alimentary plexus, incline gradually dorsalwards as 

 they proceed backwards, and join the dorsal vessel at the level of the anterior margin 

 of the intestinal caeca ; (6) in segments xi.-xiv. (and therefore in front of the intestine) 

 there are in the oesophageal wall a series of very definite and striking transverse vessels, 

 about twelve pairs per segment, joining the supraintestinal above ; the breadth of the 

 vessels is at least equal to the intervals between them. 



The postero-anterior contractility of the alimentary canal, which is so marked a 

 phenomenon in the lower forms, begins to disappear, as has been shown, in the 

 Tubificidae ; it has not, so far as I know, been described at all in the Megadrili. It has 

 not, in them, been retained in connection with the intake of fluid by the anus, since 



