THE SHORES OF BRITAIN. 105 
each shell. Instead of a collar, each leaf of the mantle 
is here fringed with a series of delicate fleshy threads, 
which secrete the exterior part of the shell, by being 
thrust out round the edge; while the whole surface 
of the mantle deposits the beautiful, rainbow-tinted, 
pearly substance with which the interior is coated. 
Instead of the fleshy belly on which the Univalves 
glide along, the Bivalves are furnished with a pecu- 
liar organ, which in some species serves the purpose 
of motion. The Oyster, however, and some other 
species, have no power of changing their position, 
but are, as it were, cemented to the rock on which 
the spawn first chanced to fall. The Mussel, again, 
is fastened, but in a different manner, being moored 
by a cable of silken threads, which it spins from its 
own body. But the Cockle, which is eaten by the 
poor on many of our shores, is enabled to move with 
considerable rapidity by means of the organ to which 
I have just alluded. It is somewhat like a tongue, 
and can assume a great variety of shapes. The 
Cockle burrows in the mud: having lengthened and 
stiffened its tongue or foot, it pushes it as far as it 
can reach into the mud; then bending the tip into a 
hook, it forcibly contracts it, and thus brings its 
body, shell and all, into the hole. The Razor-shell, 
a shell common on sandy beaches, of a long narrow 
form, has this power still more remarkably deve- 
loped. 
Many of the islands which stud the sea around the 
north and west coasts of Scotland are remarkable for 
the stern grandeur of their precipitous cliffs. One 
might almost imagine that the surges of the mighty 
