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THE TEETH OF INVERTEBRATES. 343

Some of the nudibranclis, or naked sea-snails, and cuttlefish have a sort
of spiny internal collar in the form of an oval ring, as well as a well-
marked mandible. The trumpet-conch Ym 1C4.
(^Tiitoiilum) has two heavy black spi-
nous pieces hinged above with cartilage.
The forms of the jaw are niitnerous and
afford good characters for classification,
but they all differ from the jaws of
other invertebrates, in that the motion
and action of the jaws are essentially
vertical, and not from the sides toward jaw of V(/to(/7 ,1 • 1 II • • J. 1 1* 1 pieces of which the arch is composed.
the middle, as in insects and annelids, ^
though the accessory pieces may have a lateral motion. In some of the
Glossophora there is also a gizzard, which may be supplied with small
calcareous plates or stomacholiths, recalling the " gastric teeth " of
Crustacea.
Returning to the radula, we find in the innermost extreme of the
of the radular sac a papilla which forms the matrix of the teeth
and odontophore. This latter organ, whose situation has already been
described generally, consists of a ribbon of chitin longitudinally divided
into three areas. The central area or rhachls is bordered on each side
by a margin or pleura, which in many cases is bent up on each side so
as to form a gutter, with the rhachis at the bottom. In front of the
buccal cartilage the pleurpe are much widened laterally, so as to cover
and defend the front of the cartilage. In tiie Toxoglossa the teeth are
i'&\v in number, and appear to be inserted directly on the muscular
radular floor Avithout an odontophore. The teeth are cemented to or
spring from the odontophore, in most cases having their points directed
upward and backward. They are arranged in longitudinal and trans-
verse rows, the former in straight lines ; the transverse rows being gen-
erally curved or angulated symmetrically on each side of the median
line or tooth. Any of the longitudinal rows may be absent. In a very
few genera the radula is absent or the odontophore is edentulous.
The teeth are composed of a base, a shank or stem, and a cutting edge
or point, the latter simple or variously denticulated. The base is con-
spicuous in some forms, hardly evident in others ; in some, the surface
of the odontophore is elevated into a sort of boss beneath each tooth,
and among the limpets, etc., such bosses sometimes exist without a tooth
upon them. The shank may be short or long, simple or curiously orna-
mented, or perforated. The form of the cutting points is very varied,
and they are sometimes furnished with a minute brush-like appendage.
As a rule, the carnivorous forms have simpler and more claw-shaped
teeth.
The central tooth of each transverse row is normally symmetrical,
and the succession of them forms the median longitudinal row. These
teeth are called median or rhachidian teeth. They are generally pres-
ent, but are absent in a number of genera. On each side of the median
tooth are the lateral or pleural teeth. These are asymmetrical, being
rights and lefts, and having a tendency to bend toward the median line.
The number of longitudinal rows of laterals varies a good deal. They