STRUCTURE OF THE MOLLUSCA. 193 



merit. They are very numerous, and species occur in all 

 climates. Nearly allied to the Helicina are the Bulimi, 

 Pupce, SuccinecB, Clausilice, and other genera, forming seve- 

 ral natural families. 



The Aquatic Pulinonated Gasteropoda, although they live 

 in the water, respire air, for which they come occasionally 

 to the surface. They have only two tentacula, the terres- 

 trial species having four. Among them are the Planorbes, 

 of which the tentacula are very slender, and the shell pre- 

 sents the appearance of a flat disk, the turns being rolled up 

 in the same plane; the Livuicei, which have the shell oval or 

 oblong, and the tentacula flattened and triangular ; the 

 PhyscE, similar to the Limnaei, but with the lobes of the 

 mantle capable of covering part of the shell, which is ex- 

 tremely thin; and the Auriculce, of an oval or oblong form, 

 with the columella strongly plaited or toothed. 



ORDEH IL-GASTEPtOPODA PECTINIBRMCHMTA. 



This division of the Gasteropoda, the most numerous in 

 ge-nera and species, is characterized by having a branchial 

 cavity, in which are disposed one or two branchiae, having 

 the form of membranous laminae arranged in series. This 

 cavity is placed on the back, occupies the last or largest turn 

 of the shell, and opens externally by a large slit situated 

 between the body and the edge of the mantle. The shell 

 is almost always spirally twisted, as in the snails; their head 

 is furnished with two tentacula, and two eyes, supported 

 sometimes on tubercles ; and their mouth is in the form of 

 a proboscis. In some there is a prolongation of the mantle 

 in the form of a canal, and named the siphon, by means of 

 which the animal can respire without emerging from the 

 shell. In those species which have a siphon, there is a 

 corresponding canal at the forepart of the aperture of the 

 shell. 



