Page 334 - My FlipBook

344 DENTAL ANATOMY.
are most numerous in the land-snails, and may be wholly absent or
reach into the hundreds. Outside of the lateral teeth on each side are
frequently several series of flat, plate-like, or slender spiny teeth, which
are called uncini. They too may be very numerous, especially in the
veo;etable-feedintr sea-snails, or mav be wholly absent. But in normal
cases, when one series is absent on one side of the median line it is also
absent on the other, so that the radula with respect to the teeth is bilat-
erally symmetrical. Abnormal radulse are met with where the teeth
will be deformed or asymmetrical ; in normal radulse the anterior teeth
are usually broken and worn with use, and those in the posterior extreme
are soft, light-colored, and half formed, each longitudinal row of
teeth being secreted by the same })air of the radular papilla ; if it is
abnormal at all, the abnormality extends through the whole row during
the life of the mollusk. The adult perfect teeth vary from nearly trans-
parent to an amber-yellow or reddish-brown, and sometimes the cutting
points are black. In any large whelk they are easily visible to the
naked eye ; in large cuttlefish the radula may be an inch w'ide. On
the other hand, in some minute land-shells Vertigo, etc.), where the
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whole shell is hardly bigger than a pinhead, high powers are needed to
observe them. The radula may be quite short, reduced even to a single
pair of teeth in a few cases, while in the limpets it is very long, and in
one periwinkle {Tectarius pagoda) it has been found to be seven times
as long as the len2:th of the animal's body. Such radulse are of course
always coiled up, and only the anterior portion comes into use at any
one time.
The form and arrangement of the teeth are of great use in classifica-
tion—a fact discovered by Prof. S. Loven of Stockholm in 1846. Since
this time many authors have studied them, and great advances have
thus been made in the systematic arrangement of mollusks ; but the
number of species is so great and the workers are so few that a vast
amount remains to be done before we can consider the classification of
our American species to be placed on a sound foundation. The great
development of the groups of fluviatile and land snails [Helix, Limncea,
Physa, Yiripara, Amnicola, etc. etc.) in the woods and fresh waters of
the United States puts it in the po"\ver of any one possessed of a toler-
able microsc()])e to add solid facts to the treasury of science. Trusting
that this brief survey f>f the subject may lead some reader to interest
himself in it, I add the following instructions for examining the
radula of mollusks:
In large snails the radula and buccal mass may be easily dissected
out ; in small ones the anterior part of the body, and in minute ones the
w^hole body (after breaking the shell), may be taken. With a i)air of
forceps, a test-tube, an alcohol lamp, some watch-glasses, and some
needles fastened in little wooden handles, a little caustic potash, and a
microscope, the student is prepared for w'ork.
The radula, or the part of the snail containing it, should be dropped
into al)()ut a teaspoonful of half-saturated solution of caustic potash in
water in the test-tube. This may then be gently boiled over the lamp ;
too violent boiling may spill the contents of the tube. Held in the
forceps, the tube may be moved in and out of the flame as experience