COUKSE OF THE CIRCULATION. 515 



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 diifers 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 con- 

 veyed 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 cesophageal 

 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 ossophageal 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 im- 

 perfectly 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 



2L2 



