Page 340 - My FlipBook

350 DENTAL ANATOMY.

lateral movement in connection with the vertical movement of the true
mandible.
Having indicated the character of the teeth in divers forms of mol-
luscan animals, it remains only to refer to their functions. As already
stated, they are used for masticating vegetable or tearing animal matter
on which the creatures feed, and, in some cases, as weapons of offence
and defence. They have still another use—that of drilling through hard
substances, such as the shells of other mollusks for the purpose of
devouring the inhabitant. This causes the small round holes so com-
monly seen in dead shells on the beach. This process has been
watched, and is very slow in most cases, two- or three days being
required by a Purpura to drill through a small clam-shell. Many
young oysters are annually destroyed in this way by a mollusk known
to the oyster-men as the " drill." Some of the tropical forms secrete
an acid which nuist hasten the process a good deal, but in most cases
the work is done by pure friction with the raclula in a rotary manner.
After the hole is drilled thp destroyer inserts his proboscis and sucks
the fluids of his victim. The traces of the teeth are perfectly visible
on the sides of the perforation. Their action may be watched by put-
ting a pond-snail on the glass walls of an aquarium where it has become
overgrown with green confervoid slime. A few of these snails are fre-
quently placed in aquaria for the purpose of keeping the walls clean.