Cases 97–135.
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.)
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.
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.
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.