The Pulmonata are furnished with a lung cavity in place of the ordinary gill of other Gastropods, and may be termed true air-breathers. Most of them are provided with shells, and, with the exception of the _Amphibolidæ_ never possess an operculum. They are divisible into two groups or sub-orders, _Basommatophora_ and _Stylommatophora_, characterized by a difference in the position of the eyes. The Basommatophora, including the _Auriculidæ_, _Amphibolidæ_, _Siphonariidæ_, and _Limnæidæ_, have a single pair of non-retractile tentacles, at the base of which are situated the eyes. The Stylommatophora (Land-snails, Slugs, &c.), are provided with two pairs of retractile tentacles, with the eyes at the summit of the upper pair. Over ten thousand species of Pulmonata are known. (_Basommatophora._) [Sidenote: Cases 97–98.] The first group of the aquatic air-breathers, the _Auriculidæ_, chiefly inhabit salt or brackish water. The largest forms are tropical and found at the mouths of rivers, among the roots and stems of mangrove-trees, or in damp woods near the sea. [Sidenote: Cases 98–99.] The “Limpet-Snails” (_Siphonariidæ_) seem at first sight to be out of place among the Snails and Slugs, and more nearly allied to the Rock-Limpets; but the character of the tongue (radula) and the closed respiratory cavity indicate a close relationship with the present group. [Illustration: Fig. 21. Three Rows of Teeth of the Radula of _Siphonaria_. _c_, central; _l_, lateral teeth. ] The shells of _Siphonaria_ may be known from Limpets by a slight bulging on one side, caused by a radiating groove which interrupts the muscle of attachment. They are marine, and are found on rocks between tide-marks, chiefly in tropical countries. [Sidenote: Cases 99–101.] The _Limnæidæ_ are only found in fresh water. Most of them occasionally rise to the surface to breathe, where they glide along foot uppermost, at times suspending themselves by a glutinous thread, after the fashion of a spider. All countries appear to have their peculiar species. The freshwater Limpets (_Ancylus_) live attached to stones and leaves of plants, and have not the habit of floating, but, like the rest of the _Limnæidæ_, feed on freshwater algæ, confervæ, and decayed vegetable matter. [Illustration: Fig. 22. British Pond-Snail (_Limnæa stagnalis_). 1. Upper view: _a_, foot; _b_, tentacles; _c_, eye; _d_, muzzle. 2. Lower view: letters _a_, _b_, _c_ as above; _e_, mouth; _f_, respiratory orifice. ] (_Stylommatophora._) [Sidenote: Cases 102–135.]