326 PIIYTIVOUOUS MOLLUSCA. 



modern epicures, are fonder of their food when it has ad- 

 vanced some way to putrefaction. Dr. Fleming says : — 

 " Those which are phytivorous, appear to prefer living 

 vegetables, and refuse to eat those which are dried. We 

 are not aware that putrid vegetable matter is consumed by 

 them, although many of the snails and slugs are found 

 under putrid leaves and decayed wood. In those places 

 there is shelter from the sun, together with dampness ; so 

 that it is difficult to determine whether they sojourn in an 

 agreeable dwelling or a well-stored larder."* On occasions 

 they eat voraciously ; but, when necessary, they can sustain 

 a fast longer, perhaps, than any other animated beings ; 

 snails having been kept for upwards of a year, nay for years, 

 and the Limnaeas and Planorbes for many months, without 

 any food, except that small and tenuous portion which they 

 might extract from the air or water. 



The mouth in this tribe, as in other mollusca, is always 

 anterior and terminal, with often an inferior aspect ; in 

 Doris, and Cyclostoma, and a few others, it is prolonged into 

 a sort of snout, which can be shortened or elongated to a 

 small extent ; in Aplysia and Pleurobranchus, there are 

 labial tentacula at the sides ; and it is overshadowed in the 

 Tritonia by a deeply crenate veil, which receives a very 

 remarkable developement in the Tethys, of which I have 

 already given you a figure. It is in no instance furnished 

 with the complicated retractile proboscis of the pectini- 

 branchial Zoophaga ; but, on the contrary, we very gene- 

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

 texture, f fitted for dividing their food into appropriate 

 portions. In the marine tribes there is a pair of these in- 

 struments 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 

 reticulated plates : whereas, in Tritonia, they are composed 

 of solid horn ; and, in reference to their form, Cuvier com- 

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

 blades being large, oblong, curved, deeply emarginate be- 

 hind, and partially serrulated on the upper edge, (Fig. 63, 6). 

 The slugs and snails (Pulmonifera), whether terrestrial or 

 aquatic, have a single jaw J placed on the upper side of the 

 oral aperture ; and it acts in cutting the herbage by being 



* Edinb. Encyclop. xiv. 602. 



t Doris, Pleurobranchus, and the pcctinibranchial Phytopbaga (Littorina, 

 Trocbus, Nerita, &e.) in general are exceptions. 



:|: Hook, in bis Microgruphia, has given a magnified view and description 



