MANUAL OF THE MOLLUSCA. 



PART II. 



CLASS II. GASTEROPODA. ORDER II. PULMONIFERA. 

 THIS order embraces all the land-snails and other mollusca which breathe 

 air. They are normal gasteropods, having a broad foot, and usually a 

 large spiral. shell; their breathing-organ is the simplest form of lung, and 

 is like the branchial chamber of the sea-snails, but lined with a network of 

 respiratory vessels. One large division of the land-snails is furnished with 

 an operculated shell ; the rest are in-operculate, and sometimes shell-less. 



The pulmonifera are closely related to the plant-eating sea-snails (holo- 

 stomata), through Cyclostoma, and to the nudibranches by Oncidium. As 

 a group, they are generally inferior to the sea-snails, on account of the com- 

 parative imperfection of their senses, and the union of the functions of both 

 sexes in each individual. 



SECTION A. IN-OPERCULATA. 



The typical pulmonifera vary much in appearance and habits, but agree 

 essentially in structure. Most of them have sufficiently large shells ; in the 

 slugs, however, the shell is small and concealed, or rarely quite wanting. 

 Snail-shells contain a larger proportion of animal matter than sea-shells, and 

 their structure is less distinctly stratified (p. 40). In form, these shells repre- 

 sent many marine genera. The greater part are terrestrial, only some of the 

 smaller families inhabit fresh-waters, or damp places near the sea. The res- 

 piratory orifice is small and valve-like,* to prevent too rapid desiccation in 

 the land-snails, and to guard against the entry of water in the aquatic tribes. 

 Land-snails are universally distributed ; but the necessity for moist air, and 

 the vegetable nature of their food, favour their multiplication in warm and 

 humid regions ; they are especially abundant in islands, whilst in hot and 

 desert countries they appear only in the season of rain or dews. Their 

 geological history is less complete than that of the purely marine orders j but 



* Hence they are called Adelo-pnenmona (concealed-lunged) by Gray. 



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