46 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



A third phylum, the mollusks, have developed another large group 

 of terrestrial forms, the land snails. These are composed of two con- 

 vergently developed series which are evidently separate in origin. The 

 majority of the land snails in our zone are pulmonates, hermaphroditic 

 forms without an operculum, sometimes, secondarily, entirely without 

 a shell. Toward the tropics land snails of the prosobranchiate group 

 become progressively more numerous. These are operculate snails with 

 separate sexes. Like the crustaceans, the aquatic gasteropods possess 

 a protective structure, the shell, ready to serve as a protection against 

 evaporation for the forms taking up terrestrial life. Though it is 

 usually possible for the snail to withdraw entirely into its shell, a 

 part of its body must be stretched forth for creeping and other activ- 

 ity, and this naked part of the animal is protected against evaporation 

 primarily by a coat of mucus. The land snails are thus radically dis- 

 tinguished from the arthropods and vertebrates in which the entire 

 body is covered by the protective layers. 



Snails may pass through periods of drought by closing the shell 

 with the operculum, when an operculum is present; the non-opercu- 

 lates may secrete a film of mucus which serves the same purpose; 

 others adhere by means of the foot to a stone, a tree trunk, or a leaf. 

 In dry regions snails usually come out of their shell only during the 

 fall of dew at night or during and after rains. The land snails differ 

 from their aquatic relatives by the position and number of their skin 

 glands. The gland cells are superficial in the prosobranchiates, within 

 the epidermis; in the pulmonates they are sunk deeper into the sub- 

 epithelial layer, and open externally only by means of narrow canals, 30 

 which reduces the loss of water by evaporation. Their breathing ap- 

 paratus is formed by the mantle chamber, closed outwardly except for 

 a relatively small opening, whose inner surface is only slightly in- 

 creased by the projecting walls of the blood vessels. The breathing 

 surface of the lung is notably smaller than that of the much-feathered 

 gills of the marine water-breathing snails. The richer oxygen content 

 of the air makes it possible for the pulmonate snails to exist with a 

 reduced breathing surface, but in consequence, their activity and 

 energy development are scarcely greater than those of the marine 

 snails. 



Many pulmonate snails have lost their shells in the course of their 

 evolution, in spite of the fact that the shell forms so favorable a 

 protection against evaporation. However, by this loss and by the 

 smoothing out of the visceral sac, which was formerly contained in 

 the shell, the slugs have gained a more slender form and are thus 

 better able to avail themselves of existing retreats in which they find 



