408 GASTEROPODA. 



(r), which, in turn, drives it into the aortic or arterial system of 

 vessels. The aorta, in the case before us, divides into two princi- 

 pal trunks ; of which one (TO) is directed forwards to supply the foot 

 and anterior part of the body, while the other (t) winds among the 

 mass of viscera contained 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 re-convey- 

 ed to the branchiae, there to begin again the circuit we have 

 described. 



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 branchiae to the heart by capacious veins which 

 run beneath each branchial fringe and collect it from the numerous 

 respiratory tufts ; or if, as in Doris (Jig* 185), the branchiae en- 

 circle 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. 



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

 branchiae (Jig. 189, a, b) consist of delicate lamellae minutely 

 subdivided ; 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 penetrate 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 or- 

 ganization of these Mollusca led him to the following important 

 conclusions, which are no doubt extensively applicable to the GAS- 

 TEROPODA generally: 1. That in Aplysia there are no other 

 vessels appointed to convey the blood to the branchiae than the two 

 above described. 2. That all the veins of the body terminate in 



Cuv. Memoire sur le genre Aplysia. 



