The oesophagus which succeeds the pro boscis or mouth presents nothing worthy of notice, but it is in general quite distinct from the stomach. The conformation of the latter organ varies much. Sometimes the stomach is a simple enteroid tube (as in the nereida and terebellw); sometimes it is composed of two pouches, of which the first is membranous and may be compared to a crop, while the second is muscular and is analogous to a gizzard, as, for example, in the lumbrici, thalassenue. In other cases the stomach pre sents on either side a succession of enlarge ments which have in general the form of rounded cells, but which sometimes consti tute sacs or vast and much elongated ccecums, (as in some hirudines, figs. 68 and 69.) Lastly, we may observe that these ccecums are replaced by blind canals, either simple or ramified ; thus in the arenicola, or sand worm, we find that there communicate with the second sto mach two ccecums terminated by a soft point, with thick parietes of a yellow colour ; and in the aphroditaa the stomach opens on either side into a score of membranous appendages, which commence of very contracted diameter, but afterwards insensibly become dilated and di vide into many branches : (see fig. 70, a, the retracted probos cis, b b, the ap pendages.) This type of structure leads to thatwhich is manifested in the planarim, and also approximates to what one sees in the parasitic arachnida.
The intestine which succeeds the stomach is generally narrow, and in the majo rity of the anne lida extends in a direct line to the anus. In some species, as the amphitrites, it presents a greater or less number of convolutions. There does not exist in these animals a gland which can be re garded as a liver; properly so called : the appendages which are grouped around the stomach in the arenicoke may, indeed, be biliary vessels analogous to those of insects rather than true caeca; but in the earthworms and many other annelides the bile would seem to be secreted by a peculiar organ of a yellow colour and pulpy texture, which surrounds like a sheath a great part of the digestive canal. Lastly, in certain annelida, as, for example, the thalassernce, there exists on either side of the oesophagus a small organ, which would seem to have a secretory office, and may very probably be a salivary blood in almost all the annelida differs from that of every other in vertebrate animal by its red colour ; some times, however, this fluid has scarcely a tinge. According to M. De Blainville the blood of the aphroditw is yellow, and MM. Mayor and Gosse, of Geneva, assert that the circulating fluid of the genus clepsina, one of the hirudi nidw or leech-tribe, is even altogether white. When the blood of an annelide is examined with the microscope it is seen to contain circu lar globules, but of a much larger size, and in far less number than in human blood : it coa gulates after rest like the blood of the higher animals, but it appears to contain a very small proportion of fibrine.
The blood circulates, as we have already stated, in peculiar vessels, which its red colour renders easily distinguishable.
The vascular system has been best studied in the earthworm : above the alimentary canal there runs along the entire length of the body a contractile vessel (fig. 71, a,) which is sequently dorsal, and in which the blood passes, generally from behind forwards, times in large waves, times by small quantities pelled by the successive tractions of the divisions which this vessel forms through its entire extent. A portion of the
circulating fluid then passes into another vessel (c), which originates at the anterior ex tremity of the one above mentioned, and which runs backwards along the ventral surface of the body below the nervous column, from which circumstance it has been cal led the sub-nerval vessel by Duges. But the greater part of the blood which is con tained in the dorsal vessel, in stead of following this chan nel, passes into seven or eight pairs of large lateral branches composed each of a series of dilatations or rounded ye sides (d), which are highly contractile. These ' moniliform vessels' are placed in a situation corresponding to the ovaries : they are directed downwards and open into a ventral vessel (b), which occupies the middle line of the inferior aspect of the animal, following the same track as the sub-nerval vessel, but situated less superficially. Its parietes are contractile, and it may be seen alternately dilating and con tracting simultaneously at every part along the whole of its extent. The blood flows into this ventral vessel from before backwards, and leaves it to re-enter the dorsal vessel by passing through the branches (e) which ascend perpendicularly to join the latter, on either side of the alimentary canal, which they thus embrace, and to which they furnish a great number of ramifications. The blood con tained in the sub-nerval vessel flows equally from before backwards, and ascends to re-enter the dorsal vessel by lateral channels (f), ana logous to the anastomosing vessels which we have just described, but situated more superficially than those. These superficial transverse or dorso-abdominal vessels, as they are termed by M. Dues, severally receive a large branch from their corresponding deep-seated dorso abdominal vessel, and distribute to the skin a number of ramifications which appear to be specially destined to bring the blood into contact with the oxygen necessary for respi ration.* In the genus nais the moniliform vessels, which in the earthworm perform in some degree the office of a composite heart, seem to be re placed by a single pair of wide veins, which are contractile and analogous to a divided heart, and both the superficial and deep-seated transverse vessels by which the blood ascends to the dorsal trunk seem to rise from one and the same ventral trunk ; so that the circulatory appa ratus is more simple in these annelida than in the earthworms. The same plan pervades the sanguiferous system in the other setiferous an nelidans, in which the branchim are distributed throughout the entire length of the body ; but when these organs are collected together at a determinate point of the anterior extremity of the body it is a little different. Thus in the terebellm the ventral vessel is seen to bifurcate and to form two lateral branches which have the form of an arch, and which, after having passed over the sides of the oesophagus, re-unite above that tube to form a single trunk. This trunk reaches the anterior extremity and gives origin to three pairs of primary branches, which descend to cular receptacles at the base of the branchiw, and distribute the blood to these organs.