350 MOLLUSCA. 



The Aquatic Pulmonea have only two tentacula. They come ever and anon to tlie 

 surface to breathe, so that they can only inhabit waters of inconsiderable depth : thus they 

 live in fresh waters or in brackish pools, or at least near the sides and mouths of rivers. 



There are some amongst them without a shell : such is the 



Onchidium, Cuv.* 



A large fleshy cloak, of the shape of a buckler, overlaps the foot on every side, and even covers the 

 head when this is contracted. It has two long retractile tentacula, and over the mouth a veil, sinu- 

 ated, or formed of two triangular compressed lobes. The anus and air-passage are under the hinder 

 margin of the cloak, where, a little deeper, we find also the pulmonary sac. Near them, to the right, 

 is the opening of the female organs, while, on the contrary, that of the male organ is under the right 

 tentaculura ; and these two orifices are united by a groove which runs under and along the right edge 

 of the cloak. Destitute of jaws, they have a muscular gizzard, succeeded by two membranous stomachs. 

 Several species inhabit the coasts of the sea, but always in such a situation that they are uncovered at 

 ebb tide, when they obtain the air necessary to respiration. 



The Aquatic Pulmonea, with perfect shells, have been placed by Linna;us in his genera Helix, Bulla, 

 and Voluta, whence they ought to be withdrawn. In HeUx were the two following genera, whose aper- 

 ture, as in Helix, had its inner [or pillar] margin protuberant and arcuate : — 



The Planorbis, Brug., — 

 Had already been distinguished from Helix by Bruguieres, and even previously by Guettard, because 

 the whorls of their shell, rolled up nearly on a level, enlarge insensibly, and the mouth is wider than 

 deep.f It contains a SnaU with long, slender, filiform tentacula, at the inner base of wiiich the eyes 

 are situated. It can exude, from the margin of its cloak, a copious red liquor, Mhich is not to be mis- 

 taken for its blood. The stomach is muscular, and the food vegetable, as in the Limnaee, which are 

 the faithful companions of the Planorbes in all our stagnant waters. 



The LiMN^us, Lam., 

 Were separated from the BuUmus of Bruguieres, because, notwithstanding the similarity of the shells, 

 the margin of the Limnees is sharp-edged and not reflected, and their columella has an obUque fold. 



The shell is thin : the animal has two compressed, 

 broad, triangular tentacula, with the eyes sessile at 

 their inner base. They feed upon plants and seeds ; 

 and their stomach is a very muscular gizzard, fur- 

 nished with a crop. Hermaphrodites, after the fa- 

 shion of their order, they have the female organ rather 

 widely apart from the other, — a structure which 

 compels them to copulate in such a manner that tlie 

 individual acting as a male to his mate is the fe- 

 male to a third, and from this peculiarity we occa- 

 Fig. 1C2 — Lirausa stagiiaiis. sioually find them joined together in long strings. 



They abound in stagnant waters : and they are found plentifully, as well as the Planorbes, in marly 

 or calcareous beds, which we thus discover to have been deposited from fresh water. 



The Phys^, — 

 Which were arranged arbitrarily among the Bullae, have the shell of Linmaus, but still thinner, and 

 there is no fold on the columella. The animal, when it swims or creeps, covers its shell with the two 

 pectinated lobes of the cloak : it has two long setaceous tentacula, which are bulged at the base where 

 the eyes are placed. 



The species are small, and live in clear ponds. One of them (Bulla fontinalis. Lam.), has its vvborls sinistral, 

 [and this, indeed, is the only certain character which distinguishes the genus from Limnaeus.]| 



* M.dc Blainville has changed the name Onchiilium into Peronia, 

 anii transfers the first to the Vaginulus. He places Perouia 

 amongst his Cyclfjbraiichia; but I cannot perceive any real difference 

 between their respiratory organ and that of the other Pulmonea. [As 

 this genus is not the Onchidium nf Buchanan, as Cuvier supposed, 

 M. tie Kerussac proposes to name it Onchis.^ 



t Sowerby maiutains that the shell in Planorbis is always reversed, 

 or sittislral. — Ed. 



t When the shell is oval-globose, and the cloak sufficiently ample 

 to cover it, in an expanded state, the genus is the Awphipeplen of 

 Nilson ; [and when the shell is turreted, and the cloak entire, the 

 genus is named y^ptexa by Fleming. — Ed. J 



