DR M'INTOSH ON SOME POINTS IN THE STRUCTURE OF TUBIPEX. 261 



At the posterior end of the worm it is joined by the ventral (in a similar 

 manner to that shown in Plate X. fig. 5) and proceeds forwards as a deep red 

 trunk, the depth of its colour being, perhaps, due more to the larger calibre of 

 the vessel as contrasted with the ventral, than to its thicker coats. In this 

 region, moreover, it pulsates with a swift and clear stroke from behind forwards 

 — the action being noticeable in the penultimate segment. The ventral trunk, 

 on the other hand, so far as I could observe, remained of the same calibre at 

 this part, except when affected by the wave of perivisceral fluid. In the stasis 

 following the introduction of chloroform, the dorsal vessel becomes moniliform 

 posteriorly, being constricted, apparently, by the spasm of the septal fibres ; 

 while the perivisceral corpuscles rush with great vehemence through the 

 narrowed apertures. From the colour of the central region of the last segment, 

 it would seem that, before joining, the ventral and dorsal vessels form a slight 

 plexus, and, from the vigorous motions usually occurring in this part, there 

 could be no better region for the aeration of the blood. 



In each segment two great branches pass off from the dorsal and ventral 

 vessels respectively. Towards the posterior border a large trunk (the perivis- 

 ceral) springs on each side from the dorsal, and, proceeding outwards towards 

 the body- wall, divides into numerous capillary branches, which again unite to 

 form a trunk, nearly as large as the original, that on each side enters the ventral 

 vessel. The extensive coils formed by the perivisceral branch of the dorsal 

 provide ample freedom of motion, an arrangement so necessary during the con- 

 tortions of the worm. The coils are especially distinct towards the posterior 

 part of the body. About the middle of each segment, again, the ventral vessel 

 on each side gives off a branch, which passes upwards round the intestine ; but 

 whether it terminates by anastomosing with its fellow of the opposite side, or 

 by joining the dorsal, could not be determined. Certainly no branch of any 

 size joined the dorsal in this region. 



In some views there are, besides the perivisceral branch of the ventral, one 

 or two vessels towards the anterior part of the segment, which course outwards 

 from the ventral, and anastomose on the body-wall with branches of the peri- 

 visceral. Such a branch or branches are not strictly "intestinal," for they like- 

 wise send twigs to the body- wall. I must also add that in one specimen the 

 ventral main trunk was observed to bend outwards in a simple curve, without 

 being fixed in the centre by any vessel or fibrous tissue. The "intestinal" 

 branch thus does not always attach the main trunk closely to the alimentary 

 canal. After the addition of a little aconite (which causes spasm of the fibres 

 at the septa, so as to render the worm moniliform) the dorsal and ventral 

 vessels become much contracted posteriorly, while the periviscerals remain 

 well filled, indeed, so as to constitute a broad red bar across the segment. 

 Anteriorly some of the intestinal branches are similarly distended. 



VOL. XXVI. PART II. 3 Y 



