The dorsal wall of the stomach-intestine is invaginated in nearly all terrestrial Oligochaeta to form a ridge or typhlosole. This structure is absent in Megascolex, and in the aquatic Criodrilus and semi-marine Pontodrilus, which in many respects resemble the terrestrial forms. The ridge is complicated anteriorly, simple posteriorly, and consists of all the layers entering into the wall of the intestine. In some cases (Urochaeta, Allolobophora cyanea, etc.) a longitudinal supra-intestinal vessel runs along the cavity of the typhlosole, but in Lumbricus its walls are supplied by vessels derived from the dorsal longitudinal vessel. It is kept invaginated, according to Claparede, by muscles passing across from one to the other side in each somite. The epithelium covering it internally is often found enlarged with smaller cells between the bases of the large cells. The large cells are probably thrown off into the cavity of the tract and there resolved. The same changes have been noted in the intestinal epithelium of Rhynchelmis.
The digestive tract ends with a short thin-walled rectum, which is lined by cuticle and extends only through a single somite. The anus is dorsal in Criodrilus, and there are in this worm at least seven somites with an embryonal nerve-cord traceable behind it.
A longitudinal dorsal and ventral mesentery are present in Criodrilus. The dorsal is aborted in other instances, but the ventral usually persists.
The contents of the oesophagus are acid; of the stomach-intestine alkaline. There is a diastatic and tryptic ferment, and a peptic as well, but the latter must be inactive in the living animal owing to the alkalinity of the stomach (Krukenberg).
The vascular system of the Earthworm consists of a median dorsal vessel, of a supra-neural (=ventral) vessel, and three neural vessels, two lateral and one sub-neural1. The dorsal, supra-neural, and sub-neural vessels branch anteriorly and anastomose on the pharynx. In the region of the oesophagus six pairs of dilated and pulsatile vessels or 'hearts' pass from the dorsal to the supra-neural vessel. A small seventh pair gives origin to a lateral oesophageal vessel on each side which runs forward to the pharynx. The supra-neural vessel is connected from place to place with the capillary network in the walls of the intestine, and by this means indirectly to the dorsal vessel. A vascular loop unites the dorsal to the sub-neural vessel in each somite of at least the intestinal region. The integument and nephridia are provided with a rich supply of capillaries. The blood supply of the latter is said to be connected with the supra- and lateral, neural vessels; of the former, with the dorsal and sub-neural vessel. But there is some uncertainty about these points. In Urochaeta and Criodrilus the dorsal and supra-neural vessels give origin to the integumentary capillaries. Dilatations occur in the nephridial capillaries, especially in the middle and posterior regions of the body.
Similar dilatations occur elsewhere.
1 Howes figures in his Atlas of Practical Elementary Biology (Pl. xii. fig. 2) a sub- or infra-intestinal vessel applied to the inferior aspect of the intestine. Such a vessel exists in some Poly-chaeta and Oligochaeta, but not in the common Earthworm, so far as I can find from actual observation, by dissection of fresh specimens and of mounted sections. Benham also appears to have failed to find it (Q. J. M. xxvi. p 253). It seems to me that the dark streak on the inferior aspect of the intestine which looks like a vessel is due to the attachment of the ventral mesentery in which the supra-neural or ventral vessel is suspended, and the consequent absence of chloragogen cells. Dark lines due to the same cause mark the attachments of the septa to the intestine. If the chloragogen cells are removed by a scratch, the scratched spot has a similar dark appearance.
The most remarkable variations in the circulatory system of Oligochaeta are the following. The dorsal vessel, in two sp. of Acanthodrilus from New Zealand is double from the pharynx backwards, an embryonic feature seen also in some Polychaeta: in Megascolex and Microchaeta the sections of the dorsal vessel in the anterior somites are double between the septa, single where they perforate them (Beddard, Proc. Roy. Phys. Soc. Edinburgh, 1885). In some Earthworms, e. g. certain sp. of Acanthodrilus, Urochaeta, the posterior hearts connect a supra-intestinal vessel to the supra-neural or ventral vessel; and in some instances, e. g. Urochaeta, Enchytraeidae, etc, the dorsal vessel has moniliform pulsatile dilatations, and in the Lumbriculidae blind lateral and sometimes branched processes. Integumental capillaries are wanting in aquatic Oligochaeta, except at the posterior extremity of the body in some Naidomorpha, the Tubificidae, and Criodrilus.
The blood-vessels of Lumbricus are lined by an endothelium, the cells of which vary in character in different regions, suggesting a distinction into arteries and veins. The contained liquid is coloured with haemoglobin, as in most Oligochaeta, except Aeolosoma, Chaetogastridae, and most Enchytraeidae. It contains floating corpuscles, flattened, fusiform, sometimes nearly circular, which, according to Ray Lankester, are 'the nuclei of the endothelial cells set free from the walls of the vessels.' Vejdovsky, however, points out that the Lumbricidae, like the Leeches, have minute valve-like or irregular masses of cells connected to the walls of the vessels by fine processes. They are also present but confined to the dorsal vessel in aquatic Oligochaeta. Some of these cells are set free, according to him, and form the corpuscles which are absent only in Enchytraeidae and Naidomorpha.
The coelomic fluid contains amoeboid cells, 'large colourless vacuolated corpuscles, with a ragged outline often produced into filaments and provided with a large nucleus.'
The excretory nephridia, or segmental organs as they used to be called, are found one pair to each somite throughout the whole extent of the body from the fourth somite onwards. Each organ consists of a ciliated funnel or nephrostome, a convoluted tube, and a terminal muscular duct. The funnel is bilobed, but one of its lips is very small: its free margin is formed by a single row of ciliated cells. It opens into the cavity of the somite anterior to that in which the duct opens externally \ an arrangement common to all Oligochaeta, but by no means so in Polychaeta. The tube, therefore, to which it is connected passes backwards through a septum. The convoluted tube forms typically three loops. The first portion of it is clear-walled and ciliated; the second rather wider, its cells granular, and their cilia peculiarly long; the third part is wider still, its cells coarsely granular and not ciliated. The tube appears to perforate the cells, i. e. is intra-cellular as in the corresponding region of the Leech's nephridium. The duct is long, with muscular walls, and is dilated.