74 Natural History of Molluscous Animals : — 



very remarkable developement in the Tethys, of which I have 

 already given you a figure [V. 36. ]. It is in no instance 

 furnished with the complicated retractile proboscis of the 

 pectinibranchial Zoophaga ; but, on the contrary, we very 

 generally find, within the lips, jaws of a cartilaginous or horny 

 texture *, fitted for dividing their food into appropriate por- 

 tions. In the marine tribes there is a pair of these instru- 

 ments acting horizontally ; but they differ so much in size, 

 form, and even consistence, in the different genera, that no 

 general description could be made applicable. Usually they 

 are merely oblong pieces of cartilage ; sometimes thin reticu- 

 lated plates : whereas, in Tritonitf, they are composed of 

 solid horn ; and, in reference to their form, Cuvier compares 

 them to the scissors with which sheep are shorn, the blades 

 being large, oblong, curved, deeply emarginate behind, and 

 partially serrulated on the upper edge. (Jig. 6.) The slugs 

 and snails (Pulmonifera), whether terrestrial or aquatic, have 

 a single jaw f placed on the upper side of the oral aperture ; 

 and it acts in cutting the herbage by being brought to press 

 against a mammillary eminence on the floor of the mouth ; it is 

 of a semilunar shape, hard and corneous, and either serrated 

 on the cutting edge, or armed with a single obtuse knob in 

 the centre. The tongue is a membrane roughened with 

 minute prickles, set in the most regular array, either in close 

 transverse lines, or on the angles of a network of the most 

 minute delicacy. These prickles, by pointing backwards, 

 prevent any regurgitation of the food; and, as they are 

 capable of being raised and depressed at pleasure, they must 

 tear and rasp the vegetable fibre into shreds, and prepare it 

 for an easier digestion. The shape of the tongue, and the 

 pattern in which the prickles are set, are very variable ; and 

 I know few objects which are more interesting to the micro- 

 scopical observer. It is always, as Swammerdam remarked of 

 that of Paludina vivipara, " so elegantly formed, that it can 

 scarce be exactly described, and as difficultly be represented 

 in a figure ; n £ for, indeed, the figures which have been 



* Doris, Pleurobranchus, and the pectinibranchial Phytophaga (Litto- 

 rina, Trochus, A r erita, &c.) in general are exceptions. 



f Hook, in his Micrographia, has given a magnified view and description 

 of the snail's tooth, p. 180. tab. 25. fig. 1. See also Swammerdam, Book 

 of Nature, p. 49. ; Lister, Exer. Anat. de Cochleis, tab. 2. fig. 2., and tab. 3. 

 fig. 9.; List., Conchy. Anat., tab. 1. fig. iv., and tab. 4. fig. ii. ; Cuvier, 

 Mem., pi. 2. fig. 4. 



J Book of Nature, p. 79. Cuvier asserts that the tongue of the slugs 

 and snails is not spinous, and that the food is introduced into the gullet 

 by a sort of peristaltic movement of the tongue and the buccal mass on 

 which it lies. (Mem., xi. 17.) Dr. Fleming repeats the assertion of the 



