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THE TEETH OF INVERTEBRATES. 341

coming together of the points of the five teeth. Each alveokis consists
of tM^o halves united in the middle line, and each half of an upper and
lower portion. In life the alveolus is concealed within the tissues, only
the point of the tooth projecting. The socket is interradial in position
with relation to the test of the echinus, or opposite the interambulacra
or spaces between the rows of walking suckers. Above and between
the upper ends of the alveolar pieces are certain rather thick radial
pieces called rotulce or faloes, each of which in the Echinidce bears a
bifurcated piece known as the radius (Fig. 162, d, d). In this group,
at the oral end of the ambulacra (of the interambulacra in Cidaris), are
calcified internal arched processes called auricalce, each formed of two
pieces (Fig. 161, b, b). The auriculae are supposed to be homologous
with the internal ambulacral ossicles of the starfishes and ophiurans or
brittle stars. Retractor muscles pass to the outer edge of the alveoli
from the auricula ; the former are also connected wdth transverse mus-
cular fibres. The oral framework is also provided with protractor
muscles proceeding from the alveoli to the lower edge of the corona,
besides special muscles connected with the radii.
The food of the Echinidce consists of seaweed or small shellfish and
crustaceans, or, in the case of those forms which are edentulous, of sea-
mud and coral sand, which contains much nutritive material. While
the teeth are useful in breaking up the harder parts of the food, no
grinding or true mastication is possible, as they only meet near their
sharp and slender points.
The study of this complicated and wonderful oral apparatus, which
may be easily indulged in at any watering-place by the sea, will afford
many hours of amusement and instruction to the curious student of
nature.
Among all the invertebrate animals a parallel to the variety in form'
and importance in systematic classification of the teeth of vertebrates is
alone to be found with the Mollusca, and among them only with certaim
groups.
The Mollusca have been divided into two principal groups by later-
writers—the CepJudophora or Glossophora on the one hand, and the
Acephala or Lipocephala on the other. These have a general corre-
spondence with the possession or non-possession of a "head" or its con-
comitant, a muzzle and dental apparatus. Not every species of the
many thousands which comprise the Cephalophora (whelks, snails, peri-
winkles, coat-of-mail shells, limpets, tooth-shells, sea-butterflies, nauti-
lus, squid, or cuttle- and devil-fishes) are provided with teeth, but these
special instances are the exceptions to the rule. On the other hand, no'
single member of .the Acephala (clams, oysters, mussels, cockles, fresh-
water clams, scallops, etc.) has either a head or a dental apparatus.
The apparatus, reduced to its simplest terms (Fig. 163), consists of a
tube entering the floor of the gullet in the median line behind the mouth,
called the radulav sac. The odontophore, or chitinous band upon which
the teeth are set, pointing upward and backward like the papillse on a cat's
tongue, grows out of the radular sac like a finger-nail from its sheath.
The odontophore and teeth collectively form the radula. The floor of
the sac is carried forward by natural growth in that direction, bearing