210 CIRCULATION IN GASTEROPODA. 



is more fleshy and opaque than the auricle, of an oval form, 

 and strengthened with small muscular cords. Two prin- 

 cipal arteries take their departure from it ; one of which, 

 proceeding forwards, gives branches to the stomach, oeso- 

 phagus, to the organs of generation, to both sides of the 

 back and foot, and lastly loses itself in the veil ; while the 

 other artery, directing its course backwards, is principally 

 distributed on the rectum and liver. The veins issuing 

 from this organ, and from the intestines, run in the sides of 

 the body, where, in conjunction with the veins returning 

 from the foot, the back, and the veil, they fonn ultimately 

 two main vessels to carry the blood to the branchiae ; 

 which are external, and arranged like crests, in two rows 

 along the back, the principal ornaments of the animal.* 



We may take our next example from among the terres- 

 trial mollusca. The heart of the Slug (Limax ater) is placed 

 almost in the middle of the pulmonary cavity, "included 

 in an extremely thin bag or pericardium, in whose cavity 

 there is abundance of watery moisture, as clear as the purest 

 crystal." The auricle is of a triangular figure ; the apex rest- 

 ing on the superior surface of the oval ventricle, and the much 

 dilated base receiving the pulmonary veins, which, like those 

 of the Tethys, open into it by many mouths. But the 

 peculiarity most worthy of notice in this animal is the co- 

 lour of its arteries, — an opaque and pure white, like what 

 it would be were we to suppose them filled with milk, and 

 rendered very obvious by the darkness of the ground upon 

 which the vessels trace their course ; as, for example, in 

 the intestines, which are of a dark green ; or in the liver, 

 which is of a blackish brown colour. The finest injections 

 do not produce anything, adds Cuvier, more agreeable to 

 the eye of the anatomist than the white ramifications of 

 the arteries in the Black Slug, -j- 



The most singular deviations from the normal structure 

 and disposition of the blood-vessels in the Gasteropoda are, 

 however, to be found in the celebrated Aplysia. In this 

 mollusk the great branchial vein receives the aerated blood 

 from its little tributaries, which penetrate it in such a man- 

 ner that their orifices form imperfect circles on the inner 

 surface. The vein itself runs along the convex border of 

 a crescent-shaped membrane, supporting the branchiae, and 

 opens, as usual, into the auricle ; remarkable for size and 

 the thinness of its parietes, which resemble fine gauze, the 

 very slender fleshy filaments forming a pretty network. 



* Cuv. Mollusq. vii. 11. f Ciiv. lib. cit. xi. 25. 



