MOLLUSCA. 
356 
tentacula, longitudinally cleft as in Pleurobrancluis, and at tlieir inner bases are the eyes : between 
them is a kind of proboscis, perhaps an organ of generation. There is a large concave space in the 
anterior margin of the foot, the edges of which can be drawn together like the mouth of a purse ; and 
at its bottom is a tubercle pierced with an orifice, which is perhaps the mouth, and is surmounted by 
a fringed membrane. The inferior surface of the foot is smooth, and serves the animal to crawl on, as 
in other Gasteropodes. It carries with it a hard, flat, irregularly-rounded shell, thickest in the centre, 
with sharp margins, and lightly marked with concentric striae. It was supposed at first that the shell 
was attached to the foot, but more recent observations have proved that it is upon the cloak, and in its 
usual place. 
[Tvs'o species have been discovered : one in the Indian Ocean, the other in the Mediterranean.] 
The Hetero])oda are distinguished from all other Mollusca by their foot, which, instead of 
forming a horizontal disk, is compressed into a vertical muscular lamina, which they use as a h 
fin I and on the edge of which, in several species, is a sucker in the form of a hollow cone, that 
represents the disk of the other orders. Their branchiae, formed of plumose lobes, are situ- i, 
ated on the hinder part of the back, and point forwards ; and immediately behind them are ij 
the heart and liver, of inconsiderable size, with a portion of the viscera and the interior organs f ,, 
of generation. The body, of a transparent gelatinous substance, sheathed with a muscular 
layer, is elongate, and generally terminated with a compressed tail ; the mouth has a muscular ||| 
mass and a tongue garnished with little hooks ; the gullet is very long ; the stomach thin ; | 
two prominent tubes, on the right side of the bundle of the viscera, serve as passages to the | 
excrements, and to the eggs or semen. They swim, in ordinary, in a reversed position; and 
they can inflate the body with water in a manner which is not yet well understood. 
Forskal comprised them all under his genus PterotracJiea, which it is necessary to subdivide. 
The Cabinaria, Lam., — 
Has the nucleus (formed by the heart, the liver, and organs of generation,) covered with a thin, sym- , 
THE FIFTH ORDER OF THE GASTEROPODES. 
THE HETEROPODA, Lam.* 
metrical, conoid shell, with the point curved [ 
backwards, and often raised into a crest ; under 
One species (Car. cymbimn, Lam.) inhabits the 
Mediterranean; another the Indian Ocean (Car. > 
fragilis, B. St. Vincent). The Argonauta vitrea of|i«i 
authors may be a Carinaria, but its animal is un-Mi j 
known. 1,' 
The Atlanta, Lesueur, — 
From the observations of M. Rang, should bej|[ 
small shells of the Indian Sea ; and in one of them. 
t. tour is raised into a thin crest. They are ! 
of them, Lamanon believed that he had found the originai||' 
I 
of the Ammonites. 
► M. de lilainville makes a family of this order, whkli he names 
Nectopoda, and unites them in his Nucleobranclnata with another 
family named the Pteropoda, comprisinir, however, only Limacina o 
my Pteropodes. He adds to it, upon I know not what conjecture, the 
nr. 1... 1 fni- Arconaiita beinir ar- f 
he names 
cture, the vol. v- p. 325 . — Ed. 
