GALLERY. ] 
NATURAL HISTORY. 
67 
of the neck, and the vent is on the right side of the body, near the 
head. In Patella , the gills form a complete series round the edge of 
the mantle. In Helicon the series is interrupted over the head, and 
Lepetce differ from iBoOf in the animal being destitute of any eyes. 
The family of Sea Woodlice (Chitonim:, Case 22) are as 
anomalous as the above, since instead of having a single shell, they 
have a row of shelly valves, like plate armour, arranged in a regular 
series down the middle of the back. The animal has no tentacles, but 
a membranaceous veil over the mouth; the heart is on the rectum 
behind, and the vent is in the hinder margin. The Chitons have the 
upper part of the mantle covered with scales. The Acanthopleura 
have it covered with spines or elongated scales. The Tonichia has the 
upper surface of the mantle bald and cartilaginous, and the under 
covered with a very hard striated skin. Acanthochetes is peculiar for 
having a bundle of bristles placed on each side of the valves; and 
Chitonellus and Amicula differ from it in having the valves nearly hidden 
in the mantle of the animals. The exposed part of the valves is thick, 
and generally divided into three areas; the edge which is inserted into 
the mantle is thin, notched, or denticulated; when only a small part of 
the valves is exposed, the plate of insertion is large and broad. 
The remaining orders of Gasteropodous Mollusca have been called 
Pneumobranchiata, (Case 23—26,) from their respiring free air, 
wdiich is received into a cavity between the mantle and the back, lined 
internally with numerous reticulated vessels, like the lungs of beasts and 
birds. They are mostly terrestrial, and when aquatic they come to the 
surface of the w 7 ater to respire, but they have the pow T er of suspending 
their respiration for a considerable time during the cold weather of tem¬ 
perate and the dry season of w 7 arm climates. They are all oviparous, 
but in a few 7 the eggs are hatched before they are deposited. 
The greater part of the kinds of this order have the respiratory 
cavity closed by the edge of the mantle being attached to the back of 
the neck, leaving only a small hole, covered w 7 ith a fleshy valve, for the 
entrance and escape of air. These are all hermaphrodite, and they are 
ahvays destitute of any true operculum, but the mouths of their shells 
are closed, during the period of their torpidity, with a temporary lid, 
called the epiphragma , w 7 hich is formed by the hardened juices of the 
body moulded on the surface of the contracted animal; this lid, which 
is easily removed by the pressure of the foot, w T hen its edge has been 
softened by the mucous secretions of the animal within it, is renewed 
as often as the animals require it. 
A. The more terrestrial kinds have the eyes seated at the top of the 
long cylindrical tentacles, which are placed above the true ones. The 
latter are sometimes rudimentary, or even wanting. 
In some of these the head, eye-pedicels, and tentacles, can be with¬ 
drawn under the skin, which covers them like a sheath, into the cavity 
of the body. They have a single crescent-shaped horny upper jaw, 
which is often toothed on the edge. The stomach is single and mem¬ 
branaceous. 
The family of Arions (Arionim:, Case 23) are peculiar for 
having the end of the tail furnished with a large gland, which secretes 
a quantity of mucus; this causes the end of the body to appear 
truncated. Their respiratory cavity is on the anterior part of the body, 
