AIR-BREATHING GASTEROPODS. 65 
shell-less except when very young. Eolis and Tritonia have 
been heard to make audible sounds. The eggs of Doris 
Fic. 68.—Naked-gilled mollusca, commonly called sea-slugs. D, Dorts 
pilosa; E, Eolzs coronata ; *, foot; g, breathing-gills ; ¢, tentacles. 
are incased in a ribbon and rolled up like a watch-spring ; 
360,000 young are often produced, that are at first free 
swimmers. 
AIR-BREATHING GASTEROPODS (Pudmonata). 
General Characteristics —Mollusks possessing a single 
‘lung, a chamber on the right side of the body near the 
head, lined with a membrane containing blood-vessels. 
The cavity is closed 
6) &) 
colored. : 
byavalve. The shell, 
when present, is gen- 
erally thin and deli- 
P ad ig d-Snails Fic. 69.—a@, egg capsule of a fresh-water 
(Limnaide). — These snail; 4,c, eggs, highly magnified, show- 
are the common pond- ing the young snail. 
cate, and often highly 
