t2 ZOOLOGt. 
ribbon." Familiar examples of the class are the sea-snails, 
the sea-slugs, and the genuine air-breathing snails and 
slugs. 
In the shell-less or Nudibranch mollusks, such as the 
Fig. 81.— ^oZ/s, a Nudi- Fig. 8'^.~Physa lieterostropha. Corn- 
branch ; enlarged twice, mon pond-snail. After Morse. 
.EoUs and Do7Hs and allied forms, the gills are arranged in 
bunches on the back, as seen in Fig. 81, JEolis pilata, a 
common species on the coast of New England. In Doris 
(Fig. 82) they are confined to a circle of pinnate gills on 
the hinder part of the back. 
Fig. 84. — Limnceus elodes, a common pond-snail, showing its variations. After 
Morse. 
The air-breathing mollusks {Pithnonata) are represented 
by the pond-snails, Physa (Fig. 83) and Limnmis (Fig. 
84), and the land-snails and slugs. Fig. 85 represents 
a slug suspended by a slimy thread from a twig. 
