GENERAL ZOOLOGY 



Fig. 13.14. Pulmonale e;astro- 

 pods. A, Polyoyra, a terrestrial 

 snail; note the characteristic spiral 

 growth pattern of the shell and the 

 successive lines of efrowth marking 

 periodic additions to the edge of 

 the shell. B, a giant land slug, 

 Anolimax columbianm ; the actual 

 length of this specimen was more 

 than 6 inches. The saddle-shaped 

 remnant of the shell and the open- 

 ing through which air enters the 

 mantle cavity can be clearly seen. 

 {A, photograph courtesy The Mis- 

 souri Conservaliomst; B, from A. R. 

 Mead, 1943, American Midland 

 Aaturalisl, vol. 30, reprinted by 

 permission of the author.) 



including the slugs, are called pulmonate gastropods; the marine and fresh- 

 water forms possessing gills are termed branchiate gastropods. 



In most gastropods the sexes are separate, but some species are monoecious. 

 The eggs, laid in capsules or in gelatinous masses, contain sufficient yolk to 

 sustain the developing individuals until they reach the stage of hatching. 

 In fresh-water and terrestrial forms the young appear as miniature adults; 

 in many marine snails, the sequence includes free-swimming trochophore and 

 veliger larvae. 



rhe Gastropoda illustrate particularly well the diversity of structure and 

 habitat attained by the Mollusca. Their ancestral home seems to have been 

 the ocean, since the more primitive types are marine, whereas the more 

 specialized forms occur in fresh water and on land. It appears that from the 

 marine environment .some penetrated into fresh water, and some invaded the 

 land, becoming adapted to these new habitats. The pulmonate fresh-water 



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