STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. oy 
palpi, which look like accessory gills (Fig. 18, 7, ¢). The 
ciliated arms of the brachiopods occupy a similar position 
(Figs. 4, 5, 6). The encephalous mollusca are frequently armed 
with horny jaws, working vertically like the mandibles of a 
bird ; in the land-snails, the upper jaw is opposed only by the 
denticulated tongue, whilst the limneids have two additional 
horny jaws, acting laterally. The tongue is muscular and 
armed with recurved spines (or lingual teeth), arranged in a 
great variety of patterns, which are eminently characteristic of 
the genera.* Their teeth are amber-coloured, glossy, and 
translucent ; and being siliceous (they are insoluble in acid), 
they can be used lke a file for the abrasion of very hard sub- 
stances. With them the limpet rasps the stony nullipore, the 
whelk bores holes in other shells, and the cuttle-fish doubtless 
uses its tongue in the same manner as the cat. The tongue, or 
lingual ribbon, usually forms a triple band, of which the central 
part is called the rachis, and the lateral tracts plewre, the 
rachidian teeth sometimes form a single series, overlapping 
Fig. 15. Lingual Teeth of Mollusca. 
each other, or there are lateral teeth on each side of a median 
series. The teeth on the pleursw are termed wncini; they 
are extremely numerous in the plant-eating gasteropods (Fig. 
15, A).t 
Sometimes the tongue forms a short semicircular ridge, con- 
* The preparation of the lingual ribbon as a permanent microscopic object, requires 
some nicety of manipulation, but the arrangement of the teeth may be seen by merely 
compressing part of the animal between two pieces of glass, 
{ Fig. 15. A, lingual teeth of trochus cinerarius (after Lovén). Only the median 
tooth, and the (5) lateral teeth, and (90) wnczni of one side of a single row are repre- 
sented. B, one row of the lingual teeth of cyprea europea; consisting of a median 
tooth and three uncini on each side of it. 
