192 STRUCTURE OF THE MOLLUSCA. 



over the neck, on the right side. They all live on vegetable 

 substances. Some are terrestrial, others aquatic. 



The Terrestrial Pulmonated Gasteropodous Molluscahave 

 the head furnished with four retractile tentacula, of which 

 the upper two are longer, and bear the eyes at their tips. 

 The mouth is armed with a palatal tooth, and a small tongue 

 covered with minute prominences. The body varies in form, 

 and some are naked, or provided only with an internal shell, 

 while others have an external spiral shell, of which the apex 

 is obtuse, and the aperture entire before. The naked species 

 constitute the genera Arion, Limax, Vaginula, Parmacella, 

 and others. Those having an external shell are, Vitrina, 

 Bulimus, Pupa, Vtrtiyo, Succinea, Acliatina, Helix, and 

 others. 



The slugs, Limacina, are terrestrial pulmonated Gaster- 

 opoda destitute of external shell. Their body is elongated, 

 and they have for mantle a fleshy disk, which is hardly sepa- 

 rated from the rest of the skin, and occupies the forepart of 

 the back. They are herbivorous, feeding chiefly on young 

 plants, fruits, and mushrooms. It is in the evening, or after 

 rain that they are most active, and during the day they re- 

 main concealed in holes, under stones or dead leaves, or even 

 in the ground. During winter they remain torpid, in holes 

 or crevices, or buried in the earth. 



The snails, Helicina, have a convoluted shell, sometimes 

 flattened, sometimes globose, with a roundish or crescent- 

 shaped aperture. They differ little in their organization 

 from the slugs ; only the mantle, in place of forming a con- 

 vex scutum, constitutes a large cone, twisted upon itself, 

 and covered by the shell. In summer and autumn, they are 

 very voracious, but toward the end of the latter season they 

 eat little, and, at the approach of winter, they retire to a 

 suitable place, withdraw within the shelly close its aperture 

 with membranous layers, and remain torpid until the warm 

 weather in spring excites them to activity. These Mollusca 

 have the power not only of repairuig their shells when broken 

 or crushed, but even of reproducing their tentacula and large 

 portions of the body when removed by accident or for experi- 



