(598). The alimentary canal of the Earthworm is straight and very capacious. Its great size, indeed, is in accordance with the nature of the materials employed as food; for it is generally found distended with earth; and by the older physiologists these creatures were regarded as affording proof that the nourishment of animals was not exclusively derived from animal and vegetable substances, since in this case they supposed nutriment to be obtained from matter belonging to the mineral kingdom. This supposition, however, has been long since exploded; for it is not from the earth that nourishment is afforded, but from the decaying animal and vegetable particles mixed up with the soil taken into the stomach; so that the exception to the general law of nature supposed to exist in the Earthworm has no foundation in truth. The whole intestinal tract of one of these animals is represented in fig. 1ll: it consists of a wide oesophagus which terminates in a crop-like dilatation; to this succeeds a muscular gizzard (k), and a long sacculated intestine (l l) that passes in a direct line to the anus.

(599). In the Earthworm, says Dr. Williams, the chylaqueous fluid is almost entirely suppressed, and the visceral cavity obliterated. This vulgar worm, however, does not breathe on the atmospheric, but on the aquatic principle. It dies rapidly in perfectly dry places. Its cutaneous surface is the scene of a dense plexus of blood-proper vessels. It is always enveloped in a stratum of viscid fluid, which is remarkable for the property of absorbing and dissolving atmospheric air. This air, brought thus into immediate and intimate contact with the surface of the body, operates directly upon the blood-proper circulating in the cutaneous plexus. In the Abranchiate Annelids, as in many of the Tubicola, the alimentary canal is always profusely supplied with a vascular tissue which shares in the respiratory process, and may be regarded as a species of intestinal respiration.

Viscera of the Earthworm.

Fig. 111. Viscera of the Earthworm.

(600). The circulation of the blood in the terricolous Annelidans has been the subject of much discussion, and until recently was but very imperfectly understood. In the Earthworm there are three principal trunks connected with the vascular system*, the arrangement of which is represented in the annexed diagram (fig. 112.) First, a dorsal vessel (a) runs along the whole length of the back, in close contact with the intestine (fig. 111, o o), upon which it lies; this vessel is tortuous, and exhibits constant movements of contraction and dilatation, whereby the blood is propelled in continuous undulations from the tail towards the head. Two other large vessels occupy the ventral region of the body: of these, one (fig. 112, b), which we shall call the ventral vessel, runs immediately beneath the alimentary tube, while the other, that is situated close under the skin, and consequently beneath the ventral chain of ganglia composing the nervous system, by which it is separated from the last, may be distinguished as the sub-ganglionic vessel. These three great trunks are united by important branches, and form two distinct systems, - one of which is deeply seated, being distributed to internal viscera; the other is superficial, giving off innumerable vessels to the integuments of the body, and these, by ramifying through the skin, form an extensive vascular surface adapted to respiration.

(601). The ventral vessel (b), like the dorsal (a), may be traced quite to the anterior extremity of the worm, where numerous small anastomosing branches unite the two trunks; but these inosculations are of little consequence in describing the circular movement of the blood, - a more important communication being established, through which the blood passes freely from one to the other, by the intervention of seven or eight pairs of large canals, situated in the immediate neighbourhood of the generative apparatus, with which indeed they are interwoven. Each of these voluminous vessels (d) is composed of a series of swellings, or rounded bead-like vesicles, endowed with considerable contractile power; and they form together a kind of heart, of remarkable construction, which propels the blood received from the dorsal trunk into the ventral tube (b)*.

Circulation in the Earthworm.

Fig. 112. Circulation in the Earthworm.

* M. Duges, Ann. des Sci. Nat. vol. xv.

(602). Along the rest of the body, the communication between the dorsal and ventral trunks is repeated at each ring by canals, which are much smaller than the bead-like or moniliform vessels, and have no vesicular arrangement; they (fig. 112, g and e) run perpendicularly upwards, embracing the alimentary canal, and giving off branches at right angles, which divide into innumerable ramifications, so as to cover the whole intestine with a delicate vascular network; these may be called the deep-seated abdomino-dorsal branches.

(603). The sub-ganglionic vessel (c) may be looked upon as arising from the termination of the dorsal vessel, with which it is evidently continuous at the anterior extremity of the body. At the posterior edge of every segment a delicate branch is given off from this sub-ganglionic tube (f), which, running upwards in the same manner as those derived from the ventral trunk, joins the dorsal, and receives in its course a large anastomosing branch from the deep abdomino-dorsal canal that corresponds to it. From this system of superficial vessels arises a cutaneous network, which traverses the skin in all directions.

(604). Let us now trace the blood in its circulation through this elaborate system. In the dorsal vessel (a) the sanguineous fluid passes from the tail towards the head; at the anterior extremity of the body it passes partly into the sub-ganglionic vessel (c), through the anastomosing branches, and partly into the ventral vessel (b), into which it is forcibly driven by the contractions of the moniliform canals. In both the ventral and sub-ganglionic trunks, therefore, the course of the blood is necessarily from the head towards the tail; and the circulating fluid is continually returned to the dorsal canal by the deep and superficial abdomino-dorsal vessels (e, f, g), completing the vascular circle.