Vascular system of Terebratula Chilensis: a, c, the mantle.

Fig. 259. Vascular system of Terebratula Chilensis: a, c, the mantle; d, circular canal encompassing the margin of the mantle; f, g, h, muscles of attachment; mm mm, large venous trunks in the mantle.

(1351). A little behind the point where the heart receives this channel, two aortic vessels pass off laterally. The two orifices communicating with these vessels are guarded by sphincter valves, resulting apparently from the protrusion inwards of the lining membrane. From these vessels numerous branches are given off to supply the different viscera. The peripheral portion of the circulation is composed of an extensive system of lacunes or blood-channels, originally described by Professor Huxley, which, when the specimen is in good condition, are easily traceable, the channels or spaces being then, for the most part, filled with blood-corpuscles, which give to the lacunes an opake yellowish hue, rendering them as distinct and sharp as though they had been in-jected; and as the circular points of union between the layers of membrane forming their walls are transparent, and consequently liable to appear of a darkish tint, they show like spots on a light ground, not altogether unlike the markings upon a leopard's skin. In other lights, the whole has a beautiful lace-like delicacy.

These lacunes occasionally assume a dendritic or branched character, particularly as they approach the margin of the mantle, where they become minute, and run almost parallel to each other, forming minute twigs, which pass on to the external circumference of the lobe.

(1352). The inner lacunes, or those of the inner wall or floor of the great pallial sinuses, have a very different character; they resemble, however, the lacunes in the anterior wall of the body, with which they are in communication. They are in the form of numerous narrow channels, which, anastomosing at various points, compose a network of very long transverse meshes; thus most of the channels cross the direction of the sinuses, and run parallel to each other. Hence it would seem that the walls of the body and the laminae of the pallial lobes present one great system of blood-channels or lacunes, the various parts of which freely communicate with each other, and surround all the viscera of the body.

(1353). The blood-system of the brachial apparatus next claims attention. This is beautifully developed, and presents considerable variety in the plexuses of which it is composed. The walls of the great canal, the ridge supporting the cirri, the membranes that unite the upper and lower membranes of the loop, and that which connects the spirals, all have their systems of lacunes, which intercommunicate and form the brachial system. Let us now proceed to follow the course of the blood through this complicated vascular apparatus. It has been shown that the heart is a simple, unilocular, pyriform vesicle suspended from the dorsal aspect of the stomach, and projecting freely into the perivisceral chamber; that there is neither auricle nor pericardium, unless the membrane which closely invests it can be so called; that it is hardly more complex in structure than the pulsatile vessel of the Tunicata; nay, in Lingula it scarcely at all differs from the heart of those lowly-organized mollusks. This vesicle or heart propels the blood through four arterial trunks or channels to the reproductive organs and mantle, and probably also to the alimentary tube, and is apparently assisted by four or more pulsatile vesicles in connexion with those principal trunks.

The blood thus conveyed by the genital and pallial arteries will escape by the lacunes in the membranes suspending the genitalia, into the plexus in the floor of the great pallial sinuses. Thence it will find its way into the outer lacu-nary system of the pallial lobes, and into that of the dorsal and ventral walls of the body, as well as into the lacunae of the anterior parietes.

(1354). Having saturated all these parts of the peripheral system, it will divide itself into two currents, one of which will set backwards in the direction of the membranous bands connecting the alimentary tube to the parietes, and will flow through their channels into the system of visceral lacunes, which encircle the alimentary canal, within its sheath, and which probably carry blood to the liver. This current will also supply the lacunes nourishing the muscles. The blood thus directed will reach the branchio-systemic vein, either by the great oesophageal lacunes, or through the foramina which penetrate the sides of the channel, as it runs along the dorsal ridge of the stomach.

(1355). The other blood-current will set forward in the direction of the base of the arms, and some of it will pass into these organs through their general system of lacunae; but the principal portion will be carried by the afferent brachial canal to the extensive plexus of lacunes in those parts, and will circulate in the walls of the great brachial canal. The blood will then be drawn up one side of the cirri, through the vessels (the afferent brachial arteries) originating in the great brachial plexus, and, returning down the other, will be poured into the efferent brachial canal, and thus reach the lateral efferent sinuses at the root of the oesophagus. Thence it will enter the great oesophageal lacunes, and, there meeting with the other current of returning blood from the visceral lacunae, will be carried to the heart by the branchio-systemic vein along the dorsal aspect of the stomach. Thus it will be seen that the blood finds its way back to the heart in a mixed condition. That which is conveyed by the gastro-parietal and other channels will be imperfectly aerated, having only flowed through the pallial membranes, which must be looked upon but as accessory oxygenating agents. The arms undoubtedly perform the office of gills, and are true respiratory organs.