BIVALVE MOLLUSCA. 355 
elongate, oval, the lobes of the mantle simple or fringed, divided at 
the edge into two leaves, the interior being very short, bearing 
fringes of very minute and constantly moving cilia; the exterior gill 
is united to the shell very near the edge. The opening by which 
water and food are introduced supplies the branchiz at the same 
time. The stomach consists of a white membrane, thin, opaline, 
and presenting itself in longitudinal folds ; the liver is granular, com- 
posed of greenish grains more or less deep, contained in the meshes 
of a whitish tissue forming a thickish bed, which surrounds the 
stomach, the intestines taking the direction of the median and dorsal 
line, and beneath the heart are received and terminate in a small 
appendage, floating in the cavity of the mantle near the hinge. The 
foot is, perhaps, the remarkable organ of the mussel—it is small, 
semi-lunar when not in motion, but capable of great elongation, 
resembling thus a sort of conical tongue having a longitudinal furrow 
on its side. Itis put in motion by several pairs of muscles, all of 
which penetrate and are interlaced with the tissue; behind it is the 
silky byssus. The mouth is large, and furnished with two pairs of 
soft palpi, which are pointed and fixed by their summit. At the base 
of the foot is a gland which furnishes a viscous secretion; this 
viscous liquid is organised and moulded in the groove of the foot, 
and forms a thread, which originates the byssus; this latter is a 
bundle of viscid hairs, or threads, which holds on to its shell. 
The byssus plays an important part in the organisation of the 
mussel. While the oyster remains entirely riveted to its rock, until 
torn from it by violence, the mussel moves about, and in this motion 
the byssus is an active agent. The mussel attaches its byssus to 
some fixed object, and drawing upon it, as upon a line, the shell is 
displaced. The house is drawn onwards; the animal is in motion. 
It takes no great strides, but a fraetion of an inch satisfies its desires; 
it is, however, an advance upon the oyster, and a lesson in mechanics. 
The mussel stretches out its foot, and, at the point chosen, it fixes 
on a thread of the byssus; then, withdrawing the foot suddenly, and 
hauling in the thread, the animal and shell are moved forward. 
Every time it repeats this motion it seems to attach an additional 
thread, so that at the end of the four-and-twenty hours it has used 
many inches in length of cordage. In the byssus of some mussels 
we find as many as 150 of these small threads, with which the animal 
anchors itself most securely to the rock. Aided by this cordage, the 
mussel suspends itself to vertical rocks, holding on a little above the 
surface of the water, so that the shell is smooth and polished as 
compared with the coarse and rugged shell of the oyster. 
: xX 2 
