262 COMPARATIVE ZOOLOGY. 
most part, naked Sea-slugs, a few only having a small 
shell. The feathery gills are behind the heart (whence 
the name). They are found in all seas from the arctic to 
the torrid, generally on rocky coasts. 
When disturbed, most of them draw 
themselves up into a lump of jelly 
Fig. 222.— Bulla ampul- 
la, or “ Bubble-shell ;” 
Fie. 221.—A Tritonian (Dendronotus arborescens), three - fourths natural 
British seas. size. Indian Ocean. 
or tough skin. Examples: Sea-lemon (Doris), the beauti- 
ful 77itonia, the painted olis, the Sea-hare (Aplysia), 
which discharges a purple fluid, and the Bubble - shell 
(Bulla). 
3. Pulmonates.—These air-breathing Gasteropods, rep- 
resented by the familiar Snail, have the simplest form of 
lung —a cavity lined with a delicate net-work of blood- 
vessels, which opens externally on the right side of the 
neck. This entrance is closed by a valve, to shut out the 
water in the aquatic tribes, and the hot, dry air of summer 
days in the land species. They are all fond of moisture, 
and are more or less slimy. Their shells are lighter (being 
thinner, and containing less earthy matter) than those of 
marine Mollusks, having to be carried on the back with- 
out the support of the water. Their eggs are laid singly ; 
while the eggs of other orders are laid in chains. 
They are found in all zones, but most numerous where 
lime and moisture abound. All feed on vegetable matter. 
A few are naked, as the Slug; some are terrestrial ; others 
live in fresh water. The Land-snails, represented by the 
common f/elix, the gigantic Bulimus, and the Slug (Zz- 
max), are distinguished by their four “horns,” the short 
