Page 330 - My FlipBook

uo DENTAL ANA TOMY.

smooth median plate and lateral molar-like organs, whose mimetic
resemblance to the molar teeth of some forms of Mammalia affords a
beautiful illustration of the way in Nvhich, through the selective influ-
ence of similar functions, analogous structures may be built up in organs
which have no homology whatever. Two smaller points, bicuspid in
the lobster, tricuspid in the crab, complete the calcareous apparatus ; in
the pylorus a series of fine hairs is placed, which doubtless act as a
strainer, preventing the escape of coarser particles of food until they
have been sufficiently comminuted by these grinding organs. The
" lady " in the lobster, with which children amuse themselves, is part
of this apparatus, which of course diifers in detail in different genera
and species.
The denticulations on the claws of Crustacea have of course no claim
to be considered as teeth, though they assist in breaking up the food.
Among the echinoderms, sea-urchins, starfish, crinoids, etc. certain
forms possess an apparatus commonly known as Aristotle's lantern,
which contains what may fairly be regarded as true teeth (Fig. 161).
Among the sea-urchins the Uchinidce and Ch/peastrkke possess such an
apparatus, the mouth being central ; in the Hpatangkke the mouth is
at one side, and there are no teeth. Among the other echinoderms,
the starfishes [Astei'icke) have no teeth; the brittle stars (Ophurkkc)
have short, flat, calcareous processes which are moved by muscles
and have the name of pake
Fig. 162.
angularcs. They are attached
to the mouth-skeleton, and
are supposed to be used for
mastication.
Among the recent crinoids
or sea-lilies the mouth is
closed by lobes of the peri-
some, which may contain
calcareous plates hardly to
be called teeth. The other
groups are edentulous.
The singular and remark-
able mouth-apparatus (Fig.
1 (i2) in our common sea-urchin
(rroni the same Author.) Bental system of Kc/iinnx : 1, or sea-chestnut (Echinus) has
represents three of the pyramidal pieces foriDinR the
" lantern of Aristotle," in situ: a a, cutting extremities been observed by every one
of the incisor teeth, which are of enamel-like hardness;
"', «', «', fibrous roots of the same; /* b, opposed flat sur- Avho has passed any time at
faces of the jaws ; d d, arched processes. 2, an isolated the seaside. It is frequently
pyramid : e, its e.\teriial surface ; a, same as in 1.
detached from the test of the
animal, and retains its form for some time, even while washed about by
the waves on the l)each. It is very complicated in its arrangement,
but in essentials consists of five hard, calcareous, wedge-shaped sockets
or alveoli (Fig. 162, b, h), each containing one porcelainous chisel-
shaped tootii. The teeth (Fig. 162, a, a) are, like those of rodents,
_
softer on tlie inner than on the outer side, and therefore in wearing
always j)reserve a sharp edge. The union of the alveoli produces a
pentagonal cone with its apex pointing downward, and formed by the