22 



MANUAL OP THE MOLLUSCA. 



tained between the jaws ; at otliers, 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 180 teeth in each row. 



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

 over ; it is the part of the instrument in use, and its teeth are 

 oft-en broken or blunted. The posterior part of the lingual 



Fig. 16. Tongue of the Whelk.* 



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

 ing a tube, which is presumed 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 hullidcB 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 aplysia, 

 which is a vegetable-feeder, the gizzard 

 is armed with numerous small plates 

 and spines. The stomach of some 

 bivalves contains an instrument called 

 Fig. 17. Gizzard of Bulla, t ^he - 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 mollusca (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 undatum (original), from a preparation 

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

 Z, lateral; r, rachidian. 



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

 grown specimen, with the part nearest the head of the animal downM-ards ; in the 

 front view the plates are in contact. The cardiac orifice is in the centre, in front ; the 

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



