STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 27 



comes deeper with age. In the spiral univalves, the scar is 

 less conspicuous, being situated on the columella, and sometimes 

 divided, forming two spots. It corresponds to the posterior 

 retractors in the bivalves. 



Digestive system. This part of the animal economy is all- 

 important in the radiate classes, and scarcely of less consequence 

 in the mollusca. In the ascidians (fig. 8, i\ the alimentary 

 canal is a convoluted tube, in part answering to the oesophagus, 

 and in part to the intestine ; the stomach is distinguished by 

 longitudinal folds, which increase its extent of surface ; it receives 

 the secretion of the liver by one or more apertures. In those 

 bivalves, which have a large foot, the digestive organs are con- 

 cealed in the upper part of that organ ; the mouth is unarmed, 

 except by two pairs of soft membranous palpi, which look like 

 accessory gills (fig. 18. I. t.) The ciliated arms of the brachi- 

 pods, occupy a similar position (figs. 4, 5, 6), and are regarded 

 as their equivalents*. 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 va- 

 riety of patterns, which are eminently characteristic of the 

 genera.* Their teeth are amber-coloured, glossy, and trans- 

 lucent ; and being silicious (they are insoluble in acid), they can 

 be used like a file, for the abrasion of very hard substances. 

 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 pleura, the rachidiari teeth 



* The preparation of tlie 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. 



C 2 



