276 BRANCHLE OF BIVALVES. 



turbid.* If you find a bivalve which has not these tubes, 

 or some analogous organization, you may safely conclude 

 it to be a dweller on the surface. 



In the great majority of the Bivalves the gills are in the 

 form of large semilunar or quadrangular leaves that embrace 

 the sides of the body, whence the mollusks are often called 

 the lamellibranchiate. f There is a pair of these mem- 

 branous leaves, often of unequal size, on each side, and each 

 leaf is joined to that which corresponds to it on the opposite 

 side at the dorsal margin ; but in front they are usually 

 separate : they are broad and lamelliform, are finely and 

 regularly striated across, and sometimes appear punctulated 

 in the intervals of the stria?. Each leaf, according to Blain- 

 ville, is itself formed of two layers which leave between 

 them a free space, divided by numerous triangular partitions 

 into a great number of vertical cells open to the dorsal 

 margin. These layers are constituted by two series of paral- 

 lel vertical vessels united by others which cross them ; one 

 of the series being formed by the ramifications of the bran- 

 chial artery, and the other by those of the vein. These 

 minute ramifications can be ultimately traced to two great 

 trunks which run along the back of the branchial leaf, one 

 (the arterial) trends away to the auricle of its own side, to 

 pour into it the renovated blood, while the other is the large 

 vein from which the venous branchlets have departed. J " In 



* Ann. des Sc. Nat. n. s. iii. p. 197. When the siphonal tubes are 

 disunited, the respiratory one is the longest, and this is also the case when 

 they are partially united. In a few cases, the branchial siphon is alone deve- 

 loped. 



t Bojanus, Professor at Wilna, in an elaborate memoir published in 1810, 

 and republished in the Journal de Physique, for 1819, has maintained an 

 opinion, first mooted by Mery, that these leaves have no relation to respira- 

 tion, but are ovaries, in which the eggs are maturated ; while the true lung 

 is an organ of a brown glandular appearance placed in a sac between the 

 pericardium and the supposed branchial laminte. The opinion has been 

 very satisfactorily refuted by Blainville. 



J Manuel, p. 128. Such of our readers as are interested in the structure 

 of molluscous animals, will not be displeased at the length of the following 

 extract from Cams : — " It is to be remarked, farther, of the large branchial 

 laminae of the fresh-water mussel, that both pairs consist of an intertexture 

 of vessels arranged in a rectangular latticework, and covered by a delicate 

 membrane, whilst the two external are distinguished by a structure which 

 merits a particular description. Above each external lamina of the gills is 

 a duct, proceeding from the posterior part of the foot towards the anal tube, 

 long ago described as an oviduct by Oken, and having on its lower surface 

 a long row of openings placed transversely, and forming the entrances to the 

 cells, or compartments, of the gills themselves. These compartments are 

 all arranged vertically in the gill, and separated from each other by partitions ; 

 they appear as though they originated from the mutual recession of the two 



