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, f or 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. 



PIG. 229. A Pteropod (Hy- 



FIG. 230. A Tritonian (Denlronotus arborescem). 

 British seas. 



FIG. 231 Bulla, ampul- 

 la, or "Bubble-shell;" 

 three fourths natural 

 size. Indian Ocean 



Examples : Sea-lemon (Doris), the beautiful Tritonia, the 

 painted ^Eolis, 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 



