THE MANTLE-COVERED ANIMALS. 107 
and the lustrous transparent shell of the floating 
Carinaria, and then say whether the work done by 
the mantle of the soft-bodied animals does not surpass 
that of any sculptor or artist in the world ! 
Yet this is not the chief object of the shell, which 
is meant to shield the delicate creature within, and 
does it so successfully that though the soft bodies 
of the mollusca offer the most tempting morsels to 
birds and insects on the land, and to almost all the 
inhabitants of the sea, yet, protected by their shelly 
covering they spread into every nook and corner of 
the globe, giving birth to such multitudes of young, 
that, in spite of all the havoc which thins their ranks, 
they flourish in abundance. Even the 
" Poor patient oyster where it sleeps 
Within its pearly house," 
although it is the most helpless of all the headless 
mollusca, would overspread all the deep-sea banks 
round our coast if we would let it alone. The oyster 
fishers have only to visit their well-known haunts 
about half-a-mile or a mile from the shore, in Essex, 
Kent, Wales, and elsewhere, to rake them up by 
hundreds. If you could dive down there to the bottom 
of the sea you would find the oysters cemented firmly 
to the rocks and to each other by the under part of 
the largest valve, which is cup -like in the centre 
where the body lies, while the flatter valve is gaping 
open and a stream of water is gently flowing over 
the oyster within. 
The shells stand naturally open because they have 
an elastic cushion (c) something like a thick piece of 
gutta-percha fixed within the hinge, which acts like 
