96 
NATURAL HISTORY. [NEW BUILDING. 
within its substance. They have no operculum,, and 
usually swim about the ocean, aided by the membra¬ 
naceous 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 , Case 21) 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 , Case 21,) 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 Aplysite 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 layerf 
The family of Firoles, ( Pterotracheidce , Case 21,) which 
Lamarck separated into an order under the name of Hetero - 
podes, on account of their foot being compressed into a 
rounded, erect, muscular 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, transparent, gelatinous body, which usually ends 
in a tapering compressed tail; the heart and gills are pro¬ 
truded, forming a small dorsal mass, (which has been called 
the nucleus,') suidwhich,in Pterotrachea and Firola is naked, 
but in Carinaria is covered with a very thin, keeled, 
concentrically waved, obliquely conical, compressed shell, 
having a very large triangular mouth. These animals swim 
on the calm ocean with the back downwards; the mouth 
is large, provided with a cartilaginous tongue, and armed 
