OCR Text |
Show 1885.] NEW-ZEALAND EARTHWORMS. 821 sinus.in the walls of the intestine itself; but I have little doubt, the analogy of Acanthodrilus nova zelandia, that such a sinus exists. A delicate layer of transverse muscles, continued into the typhlosole from the intestinal walls, enclosed the blood-plexus. § Vascular System. Concerning the vascular system of Acanthodrilus I have but few remarks to offer; to study the circulation thoroughly it is requisite to have living specimens. In a short paper in the ' Proceedings of the Royal Physical Society '1 I have referred to the double condition of the dorsal vessel in two of these species, so that I need do no more than briefly recapitulate the main points of that paper, which are as follows :-In the genera Megascolex and Microchata the anterior section of the dorsal vessel is formed of two separate tubes, which become united at the points where they traverse the mesenteries. In Acanthodrilus nova zelandia the separation of the dorsal vessels into two distinct tubes has gone a step further (PI. L U I . fig. 6) : the whole of the vessel except the anterior extremity lying upon the pharynx is divided into two separate tubes which only unite at each mesentery. In A. dissimilis, which agrees so closely in external as well as in anatomical characters with A. nova zelandia, the dorsal vessel is a single tube, and appears to be always so. It is not possible, however, to use this retention of an embryonic character in A. nova zelandia, which is analogous to the partial retention of the left aortic arch in the Raptores among birds, to distinguish the species absolutely from A. dissifnilis, for in one example of the former species the dorsal vessel was single. In A. multiporus the primitive double condition of the dorsal vessel is more complete still. In this species the dorsal vessel is composed of two tubes which run from end to end of the body and are not fused at the mesenteries. The dorsal vessel is connected with the ventral (supra-nervian) by a number of transverse trunks, the last pair of which are situated iu the thirteenth segment. The last four pairs arise both from the dorsal and supra-intestinal trunks, as Perrier has recorded in Ponto-drilus and other genera. The supra-intestinal trunk lies on the surface of the alimentary canal, and is concerned with the supply of the intestinal blood-plexus. In one specimen of A. nova zelandia this vessel was double; beneath the oesophagus is another longitudinal blood-vessel, which I did not observe in the intestinal region. The supra-nervian trunk is connected with the median ventral line of the intestine by a mesentery ; and in the oesophageal region the dorsal vessel, which here lies some way above the alimentary tract, is connected with it, as elsewhere, by a similar mesentery, or two, one for each dorsal ve-sel where it is double. In the thirteenth to about the nineteenth segment is a lateral vessel on either side ; it appears to arise from a transverse heart. 1 1884-85, p.424. |