STUrCTTRE AXD PHTSIOLOGT OF THE MOLLUSCA. 



21 



'pcilpi, wliich look like accessory gills (Fig. 18, Z, t), Tlie 

 ciliated arms of the bracliiopods occupy a similal' position 

 (Figs. 4, 0, 6). The encephalous mollusca are frequently armed 

 wdth 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 like a file for the abrasion of very hard sub- 

 stances. With them the limpet rasps the stony nuUipore, 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 racliis, and the lateral tracts pleurce, the 

 rachidian teeth sometimes form a single series, overlapping 



Fif'. 15. Lingual Teeth of Mollusca. 



each other, or there are lateral teeth on each side of a median 

 series. The teeth on the pleurae are termed uncini ; 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. 



t Fig. 15. A, lingual teeth of frocAus cinerarius (after Loven). Only the median 

 tootli, and the (5) lateral teeth, and (90) uncini of one side of a single row are repre- 

 sented. B, one row of the lingual teeth of cypraa europaa ; consisting of a median 

 tooth and tlu'ee laicini on each side of it. 



