26 SHELL GALLERY. 



breathers. Most of them are provided with shells, and, with the 

 exception of the Amphibolidm never possess an operculurQ. They 

 are divisible into two groups or sub-orders, Basommatophora and 

 StyJommatophora, characterized by the difference in the position of the 

 eyes. The Basommatophora, including the Auriculidm, Amphiholidce^ 

 SiphonariidcB, and Limnmidm, have a single pair of non-retractile 

 tentacles, at the base of which the eyes are situated. The Stylom- 

 matophora (Land-snails, Slugs, &c.), are provided, save in a few cases, 

 with two pairs of retractile tentacles, with the eyes at tlie summit of 

 the upper pair. Over ten thousand species of Pulmonata are known. 



{Basommatophora.) 



Q7^Qs The first group of the aquatic air-breathers, the AvricuUdce^ 



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 The "Limpet- Snails" {Siphonariidce) seem at first sight to be 



out of place among the Snails and Slugs, and more nearly allied to 



Fig. 21. 



Three Rows of Teeth of the Radula of Siphonaria. 

 c, central ; I, lateral teeth. 



the Eock-Limpets ; but the character of the tongue (radula) and 

 the closed respiratory cavity indicate a close relationship with the 

 present group. 



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. 

 Cases The Limmeidce 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. 



99-101. 



