168 ANATOMY OF THE LAMELLIBRANCHIATA. 



back of the animal, except in the oyster, contains a thickish, 

 transparent fluid. The ventricle is muscular, and is pierced 

 by the rectum, which, in some species, though not in the 

 animal whose circulation is now described, likewise traverses 

 more or less of the aorta. The ventricle gives off an anterior 

 and a posterior aorta, and a valve exists at the com- 

 mencement of each. The former runs over the liver, giving 

 hepatic arteries, and surrounding the mouth with a ring, fur- 

 nishes the labial, ovarian, intestinal, pedal, and other arteries. 

 The latter goes backwards, and furnishes, principally, the 

 muscle and mantle with arteries. The coats of the veins are 

 thin, but they are readily injected ; those of the arteries 

 thicker, but these vessels are not easily filled with mercury. 

 Blood taken from the auricles is almost colourless, 1 separates 

 on standing into a liquid and solid part, and, microscopically 

 examined, its globules, which are about a thousandth part of 

 an inch in diameter, show some appearance of movement, 

 even out of the vessels. This fact appears to have been ob- 

 served by Mayer. 2 The heart is slow in its pulsations ; they 

 are generally from twenty to thirty in the minute. 



In the Vnio and Anadonta the sinus, corresponding to that 

 of the Pecten, lies under the pericardium, receiving anteri- 

 orly large veins from the mantle, viscera, &c, and posteriorly 

 other vessels from the posterior part of the body. Part of its 

 blood goes to the branchial artery on each side, a few twigs 

 enter the auricles and the remainder goes to the excretory 

 organs. The vein of the mantle, at one extremity, is con- 

 nected with the excretory organ. This organ on each side 

 has a few small veins entering the auricles, but the mass of 

 its blood enters the branchial artery. The branches of this 

 vessel are found on the outer side of the inner branchial la- 

 mina, and on the inner side of the external one. The cor- 

 responding venous ramuscules enter three veins, one lying 

 between the two internal bronchia, which sends its blood to 

 the two others, situated at the superior margin of the external 

 lamina. These last form the auricles. There is nothing 

 remarkable in the distribution of the arteries. 



Poli found two ventricles in the Area ; there are two like- 

 wise in some other genera, the shells of which have their 

 beaks remote. There are never more than two real auricles ; 

 but dilatations at the commencement of the aorta have been 

 so called. In the Cardium echinatum, where there are two 

 posterior aorta, each at its commencement is considerably di- 



1 It is rather red in Teredo, according to Home. 



2 Mayer, Supplem. zur Lehre vom Kreiselaufe, 1827. 



