GALLERY.] NATURAL HISTORY. 123 
are small, and the vent is on the side of the body, near 
the head. 
The family of Sea Woodlice ( Chitonidce ) are as anoma¬ 
lous as the above, since instead of having a single shell, 
they have a row of shelly valves, like plate armour, ar¬ 
ranged in a regular series down the middle of the back. 
The animal has no tentacles, and the vent is in the hinder 
margin. The Tonichia has the upper surface of the man¬ 
tle 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 Auricula only differ in having 
the valves nearly hidden in the mantle of the animals. 
The remaining Gasteropodous Mollusca have been called 
Pneumobranchiata, from their respiring free air, which 
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 water to 
respire, but they have the power of suspending their re¬ 
spiration for a considerable time during the cold weather 
of temperate and the dry season of warm climates. They 
are all oviparous, but in a few 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 with a fleshy valve, for the entrance and 
escape of air. These are all hermaphrodite, and they are 
always destitute of any true operculum, but the mouths 
of their shells are closed, during the period of their tor¬ 
pidity, with a temporary lid, called the epipkragma , 
which 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, when 
its edge has been softened by the mucous secretions of the 
animal within it, is renewed as often as the animals require 
it. 
I. 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 rudiment¬ 
ary, or even wanting. 
g 2 
