MODE OF LIFE. 421 



Bulla, Aplysia, 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 

 food 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. 



(b) Gymnosomata, without mantle fold or shell in the 



adult. Closely allied to Aplysia and its allies. 



Actively carnivorous, e.g. Clio, Pneumoderma. 

 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, e.g. sea-slugs, Doris, 

 Eolis, 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 (Limax] ; the black slug (Arion) ; fresh- water 

 snails, such as Limnaa, 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 (Limncea) 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 (Patella), 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. 



