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THE TEETH OF INVERTEBRATES. 345

will soon indicate. In a few moments the soft parts disappear, leaving
the jaw and radula in the solution, which may be poured into a watch-
glass and the radula taken out on the point of a needle, washed in pure
water, and then put under the microscope. It will be found so curved,
except in very small mollusks, as to need the presence of a cover-glass
to bring it into focus : an ordinary live-box answers well. It may be
best examined in water as a medium. To get at the form and number
of the separate teeth, it will generally be necessary to tease the radula
to pieces with the points of two needles. When the radula is micro-
scopic and cannot be seen in the liquor potassse, the watch-glass may be
put on the stage and twirled round a little, when all the solid particles
will be impelled toward the centre and the radula found and picked out
under the microscope.
After drawing the various parts, so as be able to construct a diagram
of the teeth, the object should be preserved in a little tube or vial with
some weak alcohol and a tight stopper (rubber is the best), and suitably
labelled ; or it may be mounted on a slide in the usual way, avoiding
Canada balsam, which will soon make it invisible unless stained.
The number of transverse rows of the teeth is of slight importance
compared with the exact representation diagrammatically of a single
transverse row or of the median tooth and one side of the row, which
is in most cases all that is required.
The jaw is often too horny to bear much treatment with potash ; the
teeth (except in some marine forms) are much more refractory ; experi-
ence will soon guide the student, who may practise on common species
until he gains proficiency. The character of the jaw, especially of the
land-snails, is also important for classification, and it should be care-
fully delineated.
As the number of transverse rows may be large, and the number of
teeth in each row sometimes great, the total number of teeth is occa-
sionally surprising, and has been computed for some species at .from
twelve thousand to forty thousand.
To describe the teeth of mollusks in detail would require several vol-
umes,^ even in the present imperfect state of our knowledge. The
annexed illustrations will give a general idea of their character in some
of the chief groups of mollusks.^
It is a remarkable fact that if we divide the crawling mollusks, or
Gastropoda, into two great groups, one containing the hermaphrodite
and the other the unisexual forms, we shall find that in the former
{Monceca) the auditory sacs contain numerous small otoconia, and the
form of the radula is short and. broad, the pleurae imperfectly distin-
guished from the rhachis, and the teeth usually numerous and possess-
^ Much assistance may be gained from Troschel's Gebifts der Schnecken, Berlin, 1856-
80, and in tlie works of Binney, Bland, Stimpson, and others on the land- and fresh-
water shells of the United States, published by the Smithsonian Institution at Wash-
ington. Woodward's Munnxd of Eecent ami Fossil Shells may also be advantageously
consulted. These are all cheap works. Keferences to other literatin-e of the subject
may be found in the annual volumes of the Zooloyical Record, published by Macmil-
lan, London.
^ The figures are placed in the text as if fronting the observer, with their cutting
]ioints upward as in life (except Fig. 106, ; the right side of the radula is the left side
i
of the figure in each case.