120 
NATURAL HISTORY. [jEAST, ZOOL. 
substances to its outer surface, from whence they have been 
called, respectively, “ the Conchologist ” and “ the Mine¬ 
ralogist.” Some species have this laabit only in an early 
stage, others retain it during the whole period of their 
life. 
The next order of Gasteropodous Mollusca are called 
Pleueobranchiata, from having their gills 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, aided by the membranaceous append¬ 
ages 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 hat shield, as in the genera 
Bulla , Bullcea, Acer a, and Gay ter opt era; 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 BulUnoe 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 , Dolabellay and Notarchus; the latter has no 
shell, and the Aplysice emit a great quantity of a purple 
fluid. 
The family of Pterotracheidce , which Lamarck separated 
into an order under the name of Heteropodes , on account 
of their foot being compressed into a rounded, erect fin, 
with only a sucker at its hinder edge, appear to be most 
allied to these families. The viscera, as in the slugs, 
are chiefly contained in the elongated body ; the heart and 
gills are protruded, forming a small dorsal mass, (which 
has been called the nucleus ,) and which, in Pterotrachea and 
Firola is naked, but in Carinaria is covered with a very 
thin keeled subspiral shell, having a very large triangu- 
