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338 DENTAL ANATOMY.

ing in potash solution tliey can be freed with little trouble from adhe-
rent muscles oc other organic material. Chitinous substances are, how-
ever, rather difficult to stain, and in time, unless naturally colored, become
in Canada balsam almost transparent. The student who may wish to
preserve an interesting specimen in a permanent mount for the micro-
scope should bear this in mind, and, if necessary, use some other medium.
Part of the difficulty about stainiug may be met by mounting in a tinted
medium, which will then contrast with the object itself. AVith regard
to the teeth and jaws of mollusks, special details will be mentioned far-
ther on. Many of these appendages are so thick or of such contorted
form as to require a dee]) cell and low poAvers in order to bring the
wliole in focus at once. The beauty and multitudinous variety of these
organs, and the fact that they are within easy reach of anybody, make
them very attractive objects for the microscopist, who has a wide field
for investigation in their study.
In the annelids, so-called teeth occur in many groups, but, on the
whole, partake rather of the nature of jaws than teeth, though fre-
quently double on each side, or even more numerous. This group com-
prises most of the creatures commonly called worms, as well as the
leeches, etc. Their bodies, as well shown in the common earth-worm,
are divided into more or less well-defined riugs of muscular tissue,
which correspond internally to segments, often more or less partitioned
off from each other. These rings or aunulse for the most part contain
successive groups of similar organs, but the anterior segments are usually
modified to bear sjjecial organs.
In general the jaws are developed on the second or buccal segment,
or on a proboscis which is itself an ajipendage of this segmeut, and may
be protruded from the mouth to a con-
Fig. 159.
siderable distance. They are chitinous,
most commonly paired, lateral and oppo-
FiG. 160.

Nereis nwrrjaritacen, head
n-ilh ])rotrude ratus of the pharynx, Irom
the dorsal surface ( after M.
Kdwards) : A', jaws; /'.ten- a, cephalic rcRion of the medicinal
tacles ; p, palpi ; l-'c, tenta- leech (the three jaws are visible) ; //,
cular cirri (from Claus's one of the jaws isolated, with the
Zoology). finely-serrated free edge.
site, of almost infinitely varied form, resembling in a general way the
maxillae of insects, and mimicking, in miniature, combs, saws, rasps,
claws, etc. etc.
In the leeches (Hiruclince) the mouth is provided with three lenticu-
lar jaws, with the ])rojecting edges finely serrate, and having a partly
rotatory motion about a point central to the three. The medicinal