130 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



whence the common name, " sea butterflies." Many 

 have a delicate, transparent shell. The head has six 

 appendages, armed with several 

 hundred thousand microscopic suck- 

 ers a prehensile apparatus un- 

 equalled in complication. Ptero- 

 pods occur in every latitude, but 

 generally in mid-ocean, and in the 

 arctic regions are the food of whales 



FIG. 89. - A Pteropod (Hya- an( J gea birds. 

 laa tridentata) . Atlantic. 



The sea hare (Aplysia), which 



discharges a purple fluid, and the" bubble shell (Build) 

 belong here. 



The nudibranchs or 

 sea slugs are, for the 

 most part, naked mol- 

 lusks, only a few hav- 

 ing a Shell. They are FIG. 9 o.-A Tritonian (Dendronotus arbores- 



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. Ex- 

 amples : sea lemon (Doris), the beau- 

 tiful Tritonia, and the painted sEolis. 

 4. Pulmonata. These air breath- 

 ing gastropods, represented by the 

 familiar snail, have the simplest 

 form of lung a cavity lined with 

 a delicate network of blood vessels, 

 which opens externally on the right side of the neck. 

 This is the mantle cavity. The entrance may be closed 

 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 



FIG. 91. Bulla ampulla, 

 or "bubble shell"; three 

 fourths natural size. In- 

 dian Ocean. 



