* Vide Dr. Williams's Report on the British Annelida, in the Reports of the British Association for the Advancement of Science for 1851.

(579). While, however, the peripheral segment of the vascular system in the Leech exhibits proofs of great complexity, the main currents of the blood obey two leading directions. If the body of the worm be longitudinally bisected by an imaginary horizontal plane into a dorsal and ventral semicylinder, then the blood in the primary trunks of the dorsal half will move from the tail towards the head, and in the ventral half from the head towards the tail: this movement prevails equally in the great longitudinal trunks of the integuments and alimentary canal. The transverse or circular movement of the blood is performed by means of branches which run between the main longitudinal vessels: this latter system is divisible into as many portions as there are rings in the body of the worm: each segment of the body under this arrangement has its own independent circulation, transverse and longitudinal. Thus the currents describe two excentric ellipses, cutting each other at right angles. Of course, the segmental divisions of the general system communicate with each other at every part, while the longitudinal trunks are common to all the segments.

From this description it is manifestly impossible that a distinction of venous and arterial blood can exist in the circulating fluid of this Annelid; in every part of the circumference of each ring the blood is being arterialized as it is being rendered venous; the two opposite processes proceed simultaneously in the same capillary system. The blood must be, therefore, as arterial and as venous at one and the same time in the dorsal as in the ventral trunks; notwithstanding, the dorsal main is recipient, the ventral distributive of the blood: all the secondary currents converge upon the former, and emanate from the latter; the blood in both is nevertheless identical in physiological properties.

(580). In addition to the main dorsal and ventral trunks, there exist in the Leech two strong and obvious lateral trunks, one on each side (fig. 108, ee.) The branches exhibit in their walls a structure precisely the same as that which distinguishes the vascular system in every other part of the body, while the primary lateral trunks are provided with remarkable muscular parietes, their fibres being of the striped kind. The fascicle of the muscle composing the walls is arranged in a manner which is quite distinctive of, and peculiar to, this vessel; it is coiled with so much regularity as to enclose a perfect cylinder, in which the blood flows: the longitudinal fibres are all suppressed, and therefore the circular fascicles, lined within by a hyaline membrane, constitute exclusively the coat of the vessel. Such a vessel is almost unique in. structure in the animal series; but none other would perform so admirably the peculiar duties for which it is introduced into this part, obviously as a special provision. Its business is to transmit with augmented force a current of blood in a transverse direction from the side to the ovario-ute-rine organs (fig. 108, ff), which form a double longitudinal series, one on either side of the ventral mesial line in each annular segment of the body. An express branch from the latero-abdominal trunk on either side is furnished to these reproductive organs (fig. 108, h h); so that the amount of blood propelled by this vessel, measured in its totality, must be very considerable, and the quantity during the generative season must undergo great increase in consequence of the augmentation of size which at that period these organs experience. The lateral longitudinal vessel is strikingly adapted to meet such alternation of extremes: constructed of muscle, it readily yields, under the flow of the blood-tide to the organs to whose wants it ministers, and its parietes augment by accelerated nutrition during the periods of increased local determination of blood: formed of any other structure than muscle, such admirable adaptive alternations could not happen.

(581). It has been generally considered that, in the abranchiate Annelidans, the organs provided for respiration are a series of membranous pouches, communicating externally by narrow ducts, or spiracles, as they have been termed, into which aerated water is freely admitted. These sacculi, in the Leech, are about thirty-four in number, seventeen being visible on each side of the body; they are extremely vascular; and in connexion with every one of them there is a long glandular-looking appendage, represented in fig. 110, m.

Fig. 108. Diagram illustrative of the circulatory apparatus in the Leech (Hirudo medicinalis.) (After Dr. Williams.) a, great dorsal vessel; c, ventral vessel; d d, intercommunicating vessels between dorsal and ventral trunks; ee, lateral abdominal trunks; fff, ovario-uterine organs; g, vessels distributed over the caecal appendages to the stomach; hh, branches from the lateral abdominal trunks supplying the ovario-uterine apparatus.

(582). According to the views of M. Duges, which, previous to the appearance of Dr. "Williams's interesting memoir, were received with general assent, the two lateral vessels in the Leech are appropriated to the supply of this respiratory system, and in them the blood moves in a circle quite independent of that formed by the dorsal artery and ventral vein, although they all communicate freely by means of cross branches, those passing from the lateral vessels to the dorsal being called by M. Duges* dorso-lateral, while those which join the lateral trunks to the ventral canal are the latero-abdominal branches of that observer. The movement of the blood in the lateral or respiratory system of vessels is quite distinct from that which is accomplished in the dorso-ventral or systemic trunks: sometimes it passes down one of these vessels from the head towards the tail, and in an opposite direction on the other side of the body; but in a short time the movement of the currents will be seen to become completely reversed, so that an undulatory motion, rather than a complete circulation, is kept up.