506 



MOLLUSCA 



L. zonata (Girard). 

 bands: Massachusetts. 



Body reddish in color with transverse white 



ORDER 2. PULMONATA.* 



Fresh-water and land snails (Fig. 771). Small snails, in most of 

 which the shell is a simple, regular spiral. The aperture of the shell (Fig. 

 772) is more or less circular or crescentic and usually rather small, and is 

 sometimes contracted by the presence of teeth or lamellae (7). It has an 

 outer lip or peristome (4) which is either acute or expanded, and an inner 

 or parietal lip (towards the axis of the shell) which may be either thick- 

 ened or not. Arion and Limax have no outer shell, but rudiments of one 

 are embedded in the wall of the mantle, while Pliilomycus is altogether 

 shell-less. An operculum is not found, but many land snails close the 

 aperture, on beginning thei'r hibernation, by means of a partition of cal- 



Fig. 771 Fig. 772 



Fig. 771 Helix aspersa (Lang). 1, genital pore; 2, posterior tentacle, with an 

 eye at its tip ; 3, anterior tentacle ; 4, pedal gland ; 5, respiratory pore ; 6, anus : 

 7. collar. Fig. 772 Diagram of a pulmonate shell (Walker). 1, suture; 2, apex; 

 2-8, height or length of shell ; 3, spire ; 4, lip ; 5-10, width of shell ; 6, aperture ; 

 7, parietal tooth ; 9, umbilicus. 



cified slime called the epiphragm. The mantle cavity has an anterior 

 position, except in the Onchidiidae, the opening to the outside being a 

 small pore on the right side of the body (Fig. 771, 5), which can be closed 

 at will in order to keep out the water in the aquatic species and prevent 

 desiccation in the terrestrial ones. Ctenidia are not present and aerial 

 respiration is carried on by the vascular inner lining of the mantle cavity. 

 The aquatic species are no exception to this rule, but must in most cases 

 come to the surface of the water to obtain atmospheric air; young indi- 

 viduals, however, and the adults of a few species which live in the depths 

 of deep lakes, take water into the mantle cavity, and thus respire. 



The head bears either one or two pairs of tentacles. In the aquatic 

 pulmonates but one pair is present (Fig. 776), which are not hollow or 

 retractile and have a pair of eyes at their base ; while the land pulmonates 

 have two pairs of tentacles (Fig. 771), at the tips of the posterior 

 and larger pair of which are the eyes. The jaw is composed of either 



* See "Land and Fresh-Water Shells of North America," Parts 1 and 2, by W. G. 

 Binney, Smithsonian Misc. Coll., No. 143 and 194, 1865. "Manual Am. Land Shells," 

 by same, Bull. 28, U. S. Nat. Mus.. 1885. 



