GALLERY.] NATURAL HISTORY. 123 
larged in size, stones, fragments of other shells, coral, and 
other marine substances to its outer surface, from whence 
they have been called, respectively, f<r the Conchologist ” 
and* “ the Mineralogist.” Some species have this habit only 
in an early stage, others retain it during the whole period 
of their life. 
The next order of Gasteropodous Mollusca are called 
Pleurobranchiata, from their gills, which are com¬ 
posed of plates, being placed on the right side of the 
back, and covered with a thin mantle, which is sometimes 
protected by a small shell, more or less sunk within its 
substance. They have no operculum, and usually swim 
about the ocean, aided by the membranaceous appendages 
on the side of the foot. 
In some, the gills are on the side of the back, and 
covered by the mantle, as the Bulladce and Aplysiadce. 
The family of Bubble Shells, Bulladce , have the head or 
front part of the animals without any distinct tentacula, 
the eyes being placed in a flat shield, as in the genera 
Bulla, Bullcea, Acera , and Gasteroptera; the latter has 
no shell, and the sides of its head are dilated into large 
wings, by means of which it swims about in every direc¬ 
tion. The Bullcece are very voracious, and prey on shell¬ 
fish, for which purpose they are furnished with a gizzard 
covered with three shelly plates, by means of which they 
can crack the shells after having swallowed them whole. 
The Bullince have the edge of the frontal disk produced 
into lobes. 
The family of Sea Hares, ( Aplysiadce ,) so called from 
the form they assume when sitting on the rocks, have an 
elongated head and distinct tentacula; as the genera 
Aplysia , Dolabella, and Notarchus; the latter has no 
shell, and the Aplysice emit a great quantity of a purple 
fluid. The eggs are very numerous, and are deposited in 
long fibres, which are often interlaced together. The shell 
of Dolabella is hard and thick, while that of Aplysia 
scarcely consists of more than animal matter, sometimes 
strengthened by a thin calcareous internal layer. 
The family of Firoles Pterotracheidce, which Lamarck 
separated into an order under the name of Heteropodes, on 
account of their foot being compressed into a rounded, 
erect, muscular fin, with only a sucker at its hinder edge, 
g 2 
