22 



MAXIJAL OF THE IVrOLI.IJSC A . 



tained between tlie jaws ; at others, it is extremely elongated, and 

 its folds extend backwards to the stomach. The lingual ribbon of 

 the limpet is longer than the whole animal ; the tongue of the 

 whelk has 100 rows of teeth; and the great slug has 160 rows, 

 with ISO teeth in each row. 



The front of the tongue is frequently curved, or bent quite 

 oyer ; it is the j)art of the instrument in use, and its teeth arb 

 often broken or blunted. The posterior part of the lingual' 



Fig. 16. Tongue of the Wholk.* 



ribbon usually has its margins rolled together and united, form- 

 ing a tube, which is j)resunied to open gradually. The new 

 teeth are developed from behind forwards, and are brought 

 successively into use, as in the sharks and rays amongst fishes. 

 In the hullidcG the rachis of the tongue is unarmed, and the 

 business of comminuting the food is transferred to an organ 

 which resembles the gizzard of a fowl, and is often paved with 



calcareous platen, so large and strong 



as to crush the small shell- fish which 



are swallowed entire. In the cqjlysia, 



which is a vegetable-feeder, the gizzard 



is armed with numerous small j)lates 



and spines. The stomach of some 



bivalves contains an instrument called 



Fig. 17. Gizzaid of Bulla, t ^j^^ .. crystalline stylet," which is con. 



jectured to have a similar use. In the cephalopods there is a 



crop in which the food may accumulate, as well as a gizzard for 



its trituration. 



The liver is always large in the moUusca (Fig. 10) ; its secre- 

 tion is derived from arterial blood, and is poured either into the 

 stomach or the commencement of the intestine. In the nudi- 



* Fig. 16. Lingual ribbon of buccinum iindatum (original), from a preparation 

 communicated by W. Thomson, Esq., of King's College, a, anterior; ;j, posterior; 

 /, lateral ; r, rachidian. 



t Fig. 17. Gizzard of bulla lignaria (original). Front and side view of a half- 

 growD specimen, with the part nearest tlie head of the animal downwards ; in tlie 

 front view the plates are in contact. The cardiac orifice is in the centre, in front ; I'-.s 

 lyloric orifice is on tlie posterior dorsal side, near the small transverse plate. 



