Circulating Sj/stem, 39 



in the Chit6nidse each auricle has two distinct and separated 

 ventricular orifices ; gf which, according to Cuvier, there is 

 no similar example to be found in the animal kingdom. Fur- 

 ther, in these genera, the ventricle, or proper heart, is per- 

 forated by the straight gut, or, in other words, the heart 

 encircles that intestine ; a peculiarity not to be observed in 

 other Gasteropoda. 



What, however, is the exception and anomaly among Gas- 

 teropoda, becomes the usual formation in the Conchifera or 

 bivalved MoUusca ; for in by much the greater number of 

 them the gut passes through the heart, or rather, as Blainville 

 explains it, the heart is curved round the rectum, in such a 

 manner that the two extremities of its transverse diameter 

 seem to touch. All the bivalved Mollusca have also two 

 auricles ; one placed on each side of the ventricle, and open- 

 ing by a narrow neck into it. They receive the blood through 

 a branchial vein coming from each gill, and transmit it to the 

 ventricle, which is an oval or spindle-shaped bag, situated in 

 the medio-dorsal line.* From the ventricle two aortae depart : 

 a posterior and lesser one, which passes under the rectum, 

 and distributes its branches to the posterior parts of the 

 body; and an anterior one, which runs forward even to the 

 anterior adductor muscle, furnishes branches to the stomach, 

 to the liver, the foot, and adjoining parts, then curves down- 

 wards by an anastomosing branch, which follows the margin 

 of the cloak, to meet and unite with a similar branch from 

 the posterior aorta ; forming thus a great arch, of which the 

 inferior branches go to the tentacular fringe of the cloak, 

 while the others, of greater size, remount and ramify over the 

 whole surface of that organ. 



Of the centripetal system we find that the venous radicles 

 of the belly and of the anterior parts of the body unite into 

 two large trunks, which issue from the hepatic region under- 

 neath the rectum, and, after having received two veins which 

 have followed the margin of each lobe of the cloak, they open 

 into the anterior end of a sort of auricle or venous reservoir, 

 placed longitudinally below the heart in the dorsal line. 

 This reservoir receives at its posterior end two other rather 

 large veins, which have gathered the blood and brought it 

 back from the posterior parts of the body, and even from the 

 margins of the cloak. From this reservoir the branchial 

 arteries likewise originate : they are two in number, one on 



* In the oyster, " the auricle and ventricle are very thin in their coats ; 

 so much so as to make them unequal to apply force to the blood ; but the 

 ventricle is laterally connected to the great muscle, whose action will 

 accelerate the circulation." (Home\s Comp, Anatomy ^ vol.iii. p. 160.) 



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