This circulating system varies a good deal in the different orders of the Annelides, and even in different genera, but the above slight sketch may be sufficient to convey a general idea of the system, and is well exemplified in the Nereis, as represented in Plate I., Fig. 6. In the case of the animals of this genus, as shown in the above sketch, where no heart-like centre to the circulation exists, the great dorsal vessel may be likened to a right ventricle, and the ventral vessel to a left ventricle. The duty of the former being to collect the refluent blood of the system, and that of the latter to circulate it again. These two large vessels unite at both extremities, thus causing the circulation of the blood to perform a regular circle ; besides anastomosing by transverse branches in the separate segments of the body (see Fig. 6). The blood, as Siebold says, appears to be forced in the dorsal vessel by a kind of peristaltic motion from behind forwards into the ventral vessel, which in return sends it by a similar motion into the dorsal vessel. This author remarks that the blood may occasionally also pass from the dorsal to the ventral vessels directly through the transverse anastomoses; and as this is not always in the same direction, and as these transverse anastomosing vessels are very numerous, it is very difficult to distinguish between arteries and veins. Quatrefages, however, asserts that there is no difficulty in making the distinction. The blood distributed by the dorsal, he says, is arterial, that by the ventral is venous, the dorsal vessel being analogous to an aorta, and the ventral to a vena cava. The circulating system, as we have said above, varies much in different genera. As a rule, there is no heart in the Annelides; but an exception to this occurs in the case of the lug, or sand-worm (Arenicola). In this animal there is a heartcentre, which represents a true auricle that receives the blood arriving to it, and this auricle communicates with two distinct ventricles, the contractions of which drive the blood throughout the body.

Digestive System.-The digestive apparatus of the Annelides consists of the alimentary canal and its appendages. The alimentary canal extends from one extremity of the body to the other, opening at the anterior extremity by the mouth, and at the inferior extremity by the anus. It is usually straight, rarely having convolutions. It comprises the mouth, pharynx, esophagus, and intestine. The mouth is usually surrounded by a thick lip, which is always beset with vibratile cilia. Quatrefages describes the pharynx and esophagus as parts of what he calls the proboscis. The anterior portion of this is the pharynx (Plate I., Fig. 6, b). It com

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mences immediately behind the mouth, is generally large, and of about an equal calibre throughout the whole of its length. About the middle we see inserted upon it the maxilla (or teeth), which are moved by strong muscles to open or close them (Plate I., Figs. 4 and 5). These teeth are horny in texture, and vary considerably in structure in different species, sometimes even being absent altogether. They may consist of only one pair, as in Nereis (Fig. 4, d, d), and are dentated; or they may be more numerous, as in Eunice (Fig. 5); in which case some (the upper) are strong and simply curved (Fig. 5, b, b), and others (the inferior) dentated like those of Nereis (Fig. 5, c, c.)

The posterior portion of this proboscis is, properly speaking, the esophagus (Plate I., Fig. 6, c). In a state of rest it is straight, cylindrical in form, and pleated longitudinally. In some species, this proboscis, as described above, is entirely concealed within the body; in others, as in Nereis (Fig. 4), it may be protruded externally to a considerable length, and upon its surface we see numerous small denticles scattered over it.

These portions of the alimentary canal have a particular set of blood-vessels and nerves distributed to them. The intestine (Plate I., Fig. 6, d) is a continuation of the esophagus, and generally its diameter is considerably greater. In the larger number of the Errantia, this intestine is either divided by a constriction, and then a bulging out again on each segment, or it is more or less tortuous, or even spiral. It opens posteriorly, as an anus, sometimes in the last segment, at others in the pleat which separates this ring from the penultimate. It may be situated either on the back of the segment or at its termination.

The appendages to this alimentary canal are the salivary glands and the liver. The first are sometimes wanting, but when present are attached either to the pharynx or commencement of the intestinal canal. The liver, or hepatic organs, are always present, and consist of a more or less diffused coloured tissue surrounding the whole intestinal canal.

Absorption, or Absorbing System.-Annelides have no particular organs destined for this purpose; that is, they do not possess lymphatic or chyle vessels. These two systems of vessels are represented by the general cavity of the body. In all such species as have a well-developod circulating system, we find numerous venous branches returning from the intestine, which are larger than those branches that carry the blood thither. These vessels, therefore, evidently perform the function of absorbing organs.

Organs of Secretion.-The skin appears in the Annelides to be an extremely energetic organ of secretion, and perhaps this may be owing to a particular set of organs concealed in the substance of the various layers of which it is composed. With these secretions they form the tubes in which some of them dwell: whether they are solid and calcareous, as those of the Serpula and Sabellæ ; or of a parchment-like substance, as in Chatopterus, etc. They serve the purpose also of a cement for glueing together the particles of sand, debris of shells, etc., by means of which the Terebellæ, for instance, form tubes for their reception.

Organs of Respiration.-The blood, flowing in closed vessels, requires to be aërated. This aëration of the blood is accomplished generally by means of branchial organs, or it may be carried on by some points of the skin slightly modified for the purpose, or it may be by the whole surface of the body. The branchia possess a structure peculiarly adapted for this purpose. They are characterized by the possession of a single canal into which blood-vessels arrive and from which others depart, and are covered by an extremely fine epidermis, which is furnished with vibratile cilia. They are variously formed in different genera, being very long and slender in some, as in Cirratulus (Plate IV.,* Fig. 1); arbuscular, or in the form of shrubs, as in the sand-worm (Arenicola) (Plate IV.,* Fig. 2); simply pinnated, as in Eunice; or fanshaped, as in Serpula. Sometimes branchiæ are altogether absent. In that case, according to Milne Edwards, respiration is performed by the knobs or projections we frequently see attached to the feet, as in the scale-back worms (Lepidonotus, etc.); or, as M. Quatrefages suggests, by the kind of network we see on the latero-dorsal portions of each segment in the Nereides, etc. In many cases, no doubt, respiration is carried on entirely by means of the skin covering the body, which assists materially also in those Annelides which have branchiæ.

Organs of Sense: Touch.-In the Annelides the sense of touch is exceedingly delicate. The antennæ (Plate I., Fig. 4, a, b), the tentacles (Fig. 4, c, c), and the cirri, serve evidently for the purpose of exploring and examining surrounding objects. To ascertain the correctness of this statement, it is only necessary to watch the movements of these animals in life. In some of the tubicolous worms, as the Terebellæ, for instance, the cephalic cirri, which are peculiarly extensive and retractile, may be seen elongating themselves, and carefully, as it were, feeling for the various objects around them, * Plates III. and IV. will be given with subsequent papers.