BIVALVE MOLLUSCA. 355 



elongate, oval, the lobes of the mantle simple or fnnged, 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 branchiae 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. It is 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 fraction 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. 



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