Circulating System. 37 



foot, the back, and the veil, they form 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 terrestrial 

 Mollusca. The heart of the slug (Z/imax ater) is placed 

 almost on 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 

 resting on the superior surface of the oval ventricle, and the 

 much dilated base receiving the pulmonary veins, which, like 

 those of the Tethy5, open into it by many mouths. But the 

 peculiarity most worthy of notice in this animal is the colour 

 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 grounds 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 

 any thing, adds Cuvier, more agreeable to the eye of the 

 anatomist than the white ramifications of the arteries in the 

 black slug. 



The most singular deviations from the normal structure and 

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

 ever, to be found in the celebrated Aplysia. In this mollusque, 

 the great branchial vein receives the aerated blood from its 

 little tributaries, which penetrate it in such a manner 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 [walls], which resemble fine gauze, the very slender 

 fleshy filaments forming a pretty network. The ventricle is 

 oval, and its walls are also thin, although furnished with fleshy 

 columns, crossed in every direction. The aperture between 

 it and the auricle is provided with two valves, which hinder 

 any reflux of blood. The aorta proceeding from the ventricle 

 divides into two trunks ; the first, trending directly to the left, 

 pierces the pericardium, after a very short course, to enter the 

 abdomen ; the second returns at first towards the right, sends 

 off" a branch, and then leaves the pericardium also at its right 

 side. The portion enclosed in this cavity has attached to it 

 two crests composed of small vessels, which rise from the 

 trunk itself, and again reenter it, without aflbrding the ana- 

 tomist any clue whereby to guess the use of such a curious 



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