COUESE OF THE CIRCULATION. 535 



traction of the auricle forces the circulating fluid into the ventricle (r), 

 which in turn drives it into the aortic or arterial system of vessels. 

 The aorta, in the case hefore us, divides into two principal trunks, of 

 which one (n) is directed forwards to supply the foot and anterior part 

 of the body, while the other (t) winds among the mass of viscera con- 

 tained in the shell, to which it distributes its ramifications. The blood 

 thus dispersed through the system is taken up by the commencements 

 of the veins, to be reconveyed to the branchiae, there to begin again 

 the circuit we have described. 



(1416.) When the branchiae are external, and largely distributed 

 over the surface of the body, as for instance in Tritonia, the purified 

 blood is brought from the branchiaB to the heart by capacious veins 

 which run beneath each branchial fringe and collect it from the nume- 

 rous respiratory tufts ; or if, as in Doris (fig. 266), the branchiae encircle 

 the anus, a large circular vein placed at the base of the branchial 

 apparatus receives the blood and pours it into the auricle. In all cases, 

 however, the course of the blood is essentially the same, and the heart 

 is systemic. 



(1417.) In Aplysia, one of the tectibranchiate Gasteropods, the 

 branchiae (fig. 270, a, 6) consist of delicate lamellae minutely subdi- 

 vided ; and the vessel (c) which brings the blood derived from all parts 

 of the body to be distributed over the extensive surface thus formed, 

 presents a structure of no ordinary interest to the physiologist*. At 

 some distance before it arrives at the respiratory organs it divides into 

 two main branches ; and the coats of each vessel so formed appear to be 

 made up of transverse and oblique muscular bands that cross each other 

 in all directions, so as to leave between them very perceptible apertures, 

 through which injections of any kind readily escape into the abdominal 

 cavity, and, of course, fluids derived from the abdomen as easily pene- 

 trate into the interior of the veins. At some points, indeed, these veins 

 seem absolutely confounded with the visceral cavity, a few muscular 

 bands widely separated from each other, and not at all interrupting a 

 free communication, being alone interposed. The result of Cuvier's 

 anxious researches concerning this remarkable feature in the organiza- 

 tion of these Mollusca led him to the following important conclusions, 

 which are no doubt extensively applicable to the GASTEROPODA gene- 

 rally : 1. That in Aplysia there are no other vessels appointed to con- 

 vey the blood to the branchiae than the two above described. 2. That 

 all the veins of the body terminate in these two canals. Now, as their 

 communication with the abdominal cavity is evident and palpable, 

 whether we call them venae, cavce, or cavities analogous to a right 

 ventricle, or branchial arteries, for it is manifest that they fulfil the 

 functions of these three organs, the inevitable conclusion is, that 

 fluids poured into the abdominal cavity can become directly mixed with 

 * Cuvier, Memoire sur le Genre Aplysia. 



