274 
COMPARATIVE ZOOLOGY. 
mon name, “ Sea - butterflies.” Many have a delicate, 
transparent shell. The head has six appendages, armed 
with several hundred thousand micro¬ 
scopic suckers—a prehensile apparatus 
unequalled in complication. Pteropods 
occur in every latitude, but generally 
in mid-ocean, and in the arctic regions 
are the food of Whales and Sea-birds. 
2. Opisthobranchs .—These low Gas- 
aiea tridentata). Atlantic, teropods are, for the 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 or tough skin. 
Fig. 281 .—Bulla ampul¬ 
la, or “Bubble-shell 
Fig. 230.—A Tritonian ( Denironotus arborescens). three fourths natural 
British seas. size. Indian. Ocean 
Examples: Sea-lemon {Doris), the beautiful Tritonia, the 
painted jEolis, 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 is the mantle-cavity. The 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 
