On the Blood-Vase alar System of the Earthworm Pheretima, and the Course of the Circulation in Earthworms

Abstract

ABSTRACT The typical arrangement of the blood-system in Phere-tima occurs in the region of the body behind the fourteenth segment, the first fourteen segments forming the cephalized region. The main longitudinal trunks are the same as in Lumbricus, except that the lateral neurals are absent as in Allolobophora. The dorsal vessel receives two pairs of dorso-intestinals and one pair of commissurals in each segment behind the cephalized region. The intestinal blood-plexus is both an external and an internal one, and three regions can easily be distinguished. The first is internal, and extends from the fourteenth to the twenty-sixth segment ; the second is both external and internal, is co existent with the typhlosole, and extends over the larger part of the gut ; and the third is only external, and is confined to the rectal or post-typhlosolar part of the gut (last twenty-three to twenty-six segments). The commissural vessel of Pheretima is a compound vessel, and represents both the ‘dorso-sous-nervien ’ of Lumbricus and theintestino-tegumentary of Megascolex. The capillaries of the integument are not like these of Lum-bricus but like these of Moniligaster, and there is a close ’parallelism ’ between an ‘artery ’and a ‘ vein ’in the body-wall, in which the two pass into each other through a number of capillary loops. There are four pairs of ‘hearts’ which connect the dorsal with the ventral vessel, and five pairs which supply blood directly to the various organs in the cephalized region. There are two pairs of non-contractile ‘anterior loops’ connecting the lateral oesophageals with the supra-intestinals, these loops being the counterpart of the connexions of the lateral oesophageals with the dorsal and the parietal in the tenth and twelfth segments respectively of Lumbricus. The subneural vessel is absent in the first fourteen segments, and is continuous with the lateral oesophageals of the anterior region. As regards the course of circulation of the blood, the chief fact is that the dorsal vessel is wholly ‘venous’ behind the ‘hearts’ and wholly ‘arterial ’ in the region of the ‘hearts ’ and in front (the whole of the cephalized region). The examination of valves and experiments by cutting and pinching the blood-vessels in Pheretima confirm the results of Johnstone for Lumbricus as regards the course of blood in dorso-intestinals and commissurals and make Bourne’s theory untenable. The ventral vessel is the arterial trunk throughout, while the venous function of the dorsal and subneural behind is taken up by the lateral oesophageals in the cephalized region. The thin-walled and non-contractile ‘loops’ of the tenth and eleventh segments must be distinguished from the thick walled and contractile ‘hearts’ of the other cephalized segments, the ‘loops ’ being the channels for conveying blood from the lateral oesophageals to the supra-intestinals.