MOLLUSCA. 195 
tending the length of the body, free on the sides and mar- 
gin, and striated transversely. These plates are frequently 
of unequal size. The blood is brought to these by means 
of pulmonic veins, without the intervention of the heart. 
The aérated blood is transmitted to a systemic heart, con- 
sisting of one or two auricles, and a ventricle. 
The reproductive organs of the Bivalvia, hitherto exam- 
ined, consist of an ovarium occupying the sides of the body, 
and penetrating the membranes of the cloak. They appear 
to have the organs of both sexes incorporated, and to pro- 
pagate without intercourse. Lamark is disposed to con- 
sider impregnation produced by the male fluid dispersed 
through the water; a supposition unsupported even by 
analogy in the animal kingdom. Many species are ovovivi- 
parous ; in which case the eggs when ripe pass into the gills, 
where they are hatched. 
The methodical distribution of the Bivalvia appears to be 
attended with peculiar difficulties, in consequence of the 
uniformity which prevails in the structure and disposition 
of their organs. The characters furnished by the shell, 
though useful in the construction of generic as well as spe- 
cific distinctions, have been abandoned by those who prefer 
a knowledge of the structure, rather than the form of an 
animal. The characters derived from the presence of a 
byssus, a foot, or syphons, appear to be nearly of co-ordi- 
nate importance. M. Cuvier gives the preference to those 
founded upon the appearances of the syphon, by the aid of 
which the genera may be distributed into five families, an 
arrangement which, though liable to some objections, may 
be adopted with advantage. These groups, however, may 
be considered as occupying a much higher rank, and each 
as including numerous families. 
