66 
SEA-SHELLS AND THEIR MAKERS. 
shell is clasped by two of the creature’s tentacles, and it is 
said that when captured the Argonaut often disengages 
herself from the shell, leaving it to float on the surface of 
the water and to carry its burden of eggs till they are 
hatched. Another more common shell-bearing Cephalopod 
is the Spirula , which forms a small loosely coiled and 
chambered shell of pearly white, and this is embedded in 
the lower part of the animal’s body, both of males and 
females, being of use in flotation and having nothing to do 
with the protection of eggs. 
Lastly, there is the Nautilus, which actually lives in a 
shell—a large-chambered shell—the animal occupying the 
last and largest of the chambers, while those of the inner 
coils are partially filled with air or gas, giving buoyancy to 
the shell and enabling the animal to rise or sink at 
pleasure. The Nautilus usually lives on the bottom, in 
deep water, but is believed to come to the surface at certain 
seasons. 
The Gasteropoda comprise a large number of shell¬ 
making animals, and have been divided into several orders 
or tribes, each including many families and genera. Their 
shells are always univalve, consisting of one piece, which 
is generally twisted into a spiral form. The animals have 
definite heads, which carry a pair of tentacles, a pair of 
eyes, and a sort of snout or proboscis which encloses the 
mouth. Inside the mouth there is a curious dental appa¬ 
ratus, not in the form of toothed jaws, but consisting of a 
hard and horny ribbon set with rows of minute teeth, this 
ribbon working backwards and forwards over a cartila¬ 
ginous ridge or cushion. The dental ribbon or radula 
forms an interesting microscopic object, and is of import¬ 
ance to conchologists because the form and arrangement of 
the teeth vary in different genera, and they are found to 
be a useful means of classification. The teeth are siliceous, 
and it is by the scraping action of the teeth on the radula 
that the animals masticate their food. 
Some of the Gasteropoda are vegetable feeders, browsing 
on the weeds which grow in such abundance on the sea¬ 
floor and in rock-pools between tide-marks. Others are 
carnivorous, and prey either upon their vegetarian relatives 
or on the bivalve mollusca; and some are carrion feeders, 
living upon dead fish and other creatures which die and 
leave their bodies on the sea-floor. No one can pick up 
dead shells on the sea-shore without sooner or later noticing 
that many of them are pierced by a small round hole, and 
