350 



MOLLUSCA. 



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

 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 

 tentaculum ; 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 Linnaeus in his genera Helix, Bulla, 

 and Valuta, whence they ought to be withdrawn. In Helix 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 liruguieres, 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 Snail with long, slender, filiform tentacula, at the inner base of which the eyes 

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

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

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



The Limn<eus, Lam., 

 Were separated from the Bulimus 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 oblique 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 the 

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

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

 sionally 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.e, — 

 Which were arranged arbitrarily among the Bulla?, have the shell of Limnaeus, 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 whorls sinistral, 

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



Fig. lr>2 — Llmnaea stagnalis. 



« M. dc Blainville has changed the name Onchidium into Peronia, 

 and transfers the first to the Vaginulus. He places Peronia 

 amongst his Cyclobranchia ; but I cannot perceive any real difference 

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

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

 M. de Ferussac proposes to name it Onchii.) 



t Sowerby maintains that the shell in Piauorbis is always reversed, 

 or sinistral. — Ed. 



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

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

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

 genus is named Aplexa by Fleming. — Ed ] 



