MODE OF LIFE. 421 
Bulla, Aplysta, Dolabella, Umbrella. The Tectibranchiata 
also include the Pteropoda, the winged snails or sea-butter- 
flies, which have become much modified for pelagic life. 
They have a secondarily acquired bilateral symmetry, and- 
swim by two large lateral lobes of the foot. They often 
swim actively in shoals, and occur in all seas. They afford 
tood for whales, etc., and the shells of some are abundant in 
the ooze. They include— 
(a) Thecosomata, with mantle fold and shell, diet of 
minute animal or vegetable organisms, closely related 
to Bulla and its allies. 
Examples.—Hyalea, Cymbulia. 
(4) Gymnosomata, without mantle fold or shell in the 
adult. Closely allied to Aflys¢a and its allies. 
Actively carnivorous, ¢.g. Clio, Pueumoderma, 
B. Nudibranchiata. Shell, mantle fold, and true gill are absent ; 
various forms of ‘‘ adaptive gills” may be present, or there 
may be no special respiratory organs, ¢.g. sea-slugs, Dords, 
Lolis, Dendronotus (Fig. 224). 
Order 4. PULMONATA 
The visceral loop is short and untwisted, gills are absent, and the 
mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail 
(Helix); the grey slug (Zzmax) ; the black slug (dréon) ; fresh-water 
snails, such as Lemnea, Planorbis, and Ancylus. 
Mode of life.—From the number of diverse types which 
the class includes, it is evident that few general statements 
can be made about the life of Gasteropods. We are safe in 
saying, however, that though the majority are sluggish when 
compared with Cephalopods, they are active when compared 
with Lamellibranchs. ; 
The locomotion effected by the contractions of the 
muscular foot is usually a leisurely creeping, but there are 
many gradations between the activity of Heteropods in 
open-sea, the gliding of fresh-water snails (Zzmuca) foot 
upwards across the surface of the pool, the explorations of 
the periwinkles on the sand of the shore, and the extreme 
passivity of limpets (/a¢e//a), which move only for short 
distances at a time from their resting-places on the rocks. 
The number of terrestrial snails and slugs, breathing the 
air directly by means of a pulmonary chamber, is estimated 
at over 6000 living species, while the aquatic Gasteropods 
are reckoned at about 10,000, most of which are marine. 
