ANATOMY OF THE LAMELLIBRANCHIATA. 169 



lated, and the cavities thus formed have strong muscular co- 

 lumns on their internal parietes. 



The communication between the auricles, effected by a 

 transverse vein in the Pecten, Spondylus, fyc, is more com- 

 plete in the oyster, where they are united into one, but there 

 are still two auriculo-ventricular openings. 



The veins, then, do not all enter the branchial artery, some 

 joining the branchial vein, where it forms the auricles. In 

 the Pecten we see the visceral blood circulating through the 

 excreting organs, which return it to the branchial arteries. 

 The veins between the excreting organs and the sinus, may 

 be the channels by which the former receive blood from, or 

 remit it to the latter. The first supposition is perhaps the 

 correct one, and in this case, this part of the circulation in 

 the Anadonta, &c, only differs by the visceral blood entering 

 the sinus before it circulates through the excreting organs ; 

 and by more of the blood of the sinus going to them. This 

 distribution is something like a portal system. There is a free 

 passage from the veins of the mantle into the auricles and 

 sinus in the Pecten, and into the sinus and tissue of the ex- 

 cretory organs in the Anadonta, &c. * 



RESPIRATORY SYSTEM. 



As is known to zootomists, the branchiae of the Lamelli- 

 branchiata are ciliated for the purpose of exciting currents in 

 the water. In the Monomyaria and Arcacece there are no 

 orifices or siphons to the mantle for the inlet and exit of the 

 water &c. In the Pecten, Spondylus, and Lima the branchiae 

 of each side are situated on a triangular membrane, at a dis- 

 tance from those of the other. The two branchice of the same 

 side are not distinct from each other, and their processes are 

 disunited, and do not form a continuous membrane as is ge- 

 nerally the case, but are kept in contact with each other by 

 lateral processes. In the oyster the branchice have not only 

 their processes conjoined into a membrane, but the several 

 laminae are united at their bases. In the Area, Pectunculus, 

 &c. the branchiae of each side are separate from those of the 

 other; as they are in Modiola, Mytilus, Lithodomus, and 

 other genera; but in these latter, the water enters by the pos- 

 terior fringed extremity of the mantle, and makes its exit by 

 the separate orifice situated higher up. There is a valve be- 



1 Treviranus considers the blood from the branchial veins to pass through 

 the excretory organs before it enters the auricles. Vanderhoeven was aware 

 that the venous blood circulated through them, in opposition to the erro- 

 neous opinion of the former, according to Prof. Grant. Vid. Lancet. 



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