MOLLUSKS SPECIALISTS IN SECURITY 



643 



Chap. 31 



at higher temperatures in the south. In Long Island Sound, the season is late 

 June to September; in Chesapeake Bay, May to October; in Puget Sound, 

 May to October. 



Class Gastropoda — Snails and Slugs 



Gastropods are distributed in almost every part of the earth — land and fresh- 

 water snails, great numbers of marine snails including the huge whelks and 

 conchs and the limpets. The soft naked land slugs are limited to moist places; 

 the equally naked nudibranchs are marine. There are some 30,000 living 

 species of gastropods and many more that exist only as fossils, among them 

 limpets of millions of years ago. 



Structure. The common edible garden snail (Helix aspersa) is often taken 

 as a type (Fig. 31.11). This snail moves about on its fleshy foot leaving a trail 

 of mucus from the gland within it. On the prominent head there are two pairs 

 of tentacles, the shorter pair sensitive to smells, the longer one to light. The 

 single coiled shell is secreted by a mantle as in other mollusks. The organs of 

 the body are crowded within it, a complicated mass of twisted viscera including 

 a complete male and female reproductive system, and a digestive tube begin- 

 ning at the mouth, twisting upward into the spire and turning back toward 

 the head to end in the anal opening (Fig. 31.3). One section of the mantle is 

 an air sac whose walls are supplied with blood vessels and blood pumped by 

 the heart; thus it functions as a lung. Most fresh-water snails come to the sur- 

 face and take air into the air sac or breathe through their skin; the majority 

 of marine snails breathe by gills. 



Activities and Functions. The snail's shell is a house into which it retreats. 



Heart 

 Pulmonary vein 

 Mantle cavity 



Eyes in upper tentacles; 



lower tentacles sensitive to contact. 



Genital pore 



Pedal ganglion 



Mouth 



Buccal mass 

 with rasping 

 tongue 



Fig. 31.11. The form and part of the general anatomy of a snail; the right side 

 with the shell removed; the reproductive systems, male and female, are not shown. 



