212 CIRCULATION IN THE BIVALVES. 



cular vessels for the nutritive fluid ; and we have already 

 seen a trace of it in the Cephalopoda, where the venae cavse 

 and the abdominal cavity communicate together through 

 the medium of certain spongy glands.* 



In the genus Onchidium, a naked mollusk, the venae cavag 

 exhibit a formation in some respects similar to that in Ap- 

 lysia ; but I pass over this, to notice another sort of devi- 

 ation from the common, in the Ear Shell (Haliotis), and 

 in some simple univalves, as, for example, in Fissurella. 

 The heart in these genera is provided with two auricles ; 

 one of which receives the vein carrying the purified blood 

 from the right branchiae, and the other that from the left. 

 The auricles open into the ventricle, each by a single and 

 generally narrow orifice ; but in the Chitonidae each auricle 

 has two distinct and separated ventricular orifices ; of which, 

 according to Cuvier, there is no similar example to be found 

 in the animal kingdom. -j- Further, in these genera the 

 ventricle, or proper heart, is perforated 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- 

 teropods, becomes the usual formation in the Conchifera or 

 bivalved mollusca ; for in by much the greater number of 

 them the gut passes through the heart, or rather, as Blain- 

 ville explains it, the heart is curved round the rectum, in 

 such a manner that the two extremities of its transverse 

 diameter seem to touch. The blood, collected from all parts 

 of the body by the ramifications of the venous system, is 

 poured by four principal trunks into a venous reservoir or 

 auricle analogous to that which is found in the Sepia, and 

 situated under the heart on the mesial line at the root of 

 the branchial leaflets. From every point of this venous 

 reservoir there depart a great number of vessels, which may 

 be regarded as branchial arteries ; and these, after having 

 formed a considerable vascular net-work (increased by some 

 veins composing, or only surrounding, a brown glandular 

 organ on each side), proceed lastly to re-imite in the bran- 

 chial longitudinal arteries which run along the back of the 

 branchial lamellae, and then subdivide there in the manner 

 they do in every respiratory organ. It is from the extremi- 

 ties of these subdivisions of the branchial arteries, that the 



* Mcm.ix. 14. — Mr. Owen has found the same structure to exist in the 

 Pearly Nautilus; and he suspects "that it may be more generally found on 

 a further and more diligent investigation of the venous system in this re- 

 markable class of animals." — Mentoir, 28. t Mem. xviii. 25. 



