60 
number of teeth in a row, and the number of rows in the longitu- 
dinal series, also vary greatly. The number of teeth in a row 
varies from 3 to 180, and the number of rows from (say) 2 24 to 
160 or 170. 
The arrangement of the teeth is such as to form a triple band 
longitudinally, the central line being the rachis, carrying some- 
times a single tooth, and sometimes one or more central teeth 
flanked by ‘lateral teeth; while the marginal tracts are termed 
pleure, and the teeth ered thereon are calted uncint. 
The tectibranchiate sea-slugs exhibit great variety of dentition. 
The Aplysia (sea-hares) have a large, and beautiful odontophore, 
the anterior portion of which is expanded on a muscular organ in 
the mouth, the hinder and lower portion being tubular, and hidden 
away asa reserve to be brought into use when the older parts 
become worn away. 
The Aplysie feed on both animal and vegetable substances, 
and are provided with a pair of horny cutting jaws, and with a 
gizzard armed with numerous small plates and spines. 
The Bullide also have a gizzard, the calcareous plates of 
which serve to crush small mollusca which are swallowed entire 
by the carnivorous Bulla, whose odontophore lacks the central | 
(rachidean) teeth. 
Among the Nudibranchiata some of the Doridide have a Bioad 
tubular odontophore, similar to that found in Aplysia, carrying 
lateral rows of large hooked teeth; while others have a narrow 
tongue bearing sometimes only two rows of teeth. Doris 
tuberculata affords a very beautiful example of the broad tubular 
odontophore. The lovely Holidide have a narrow lingual ribbon, 
and, in some species, the transverse plates have each but a single 
tooth ; in others the transverse plates from arched rows of spinous 
teeth set like projecting combs. 
In this family the tongue is attached to a large fleshy cushion - 
enclosed within the concavity of a pair of large horny jaws, which 
are united by a strong ligament forming a hinge joint on the upper 
margin, so that the jaws work vertically. 
The anterior edge of each jaw is, as it were, recurved, and 
then produced outward, so as to form a cutting edge. The whole 
of these organs are placed within the head of the animal, and 
communicate with the opening of the mouth by a tube, within 
which is acircular band of strong muscles. This, when pushed ~ 
forward, carries with it the mouth and jaws for the purpose of — 
securing the prey, which consists of small marine animals. The 
larger Zoophytes are also often attacked by the Kolids. 
The next Order of Gasteropods, Prosobranchiata, is divided 
into two sections. Holostomata; Univalve molluses having the 
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