GASTROPODS. 3 8 9 



piscinalis occur in places throughout the British Isles. The shells of the 

 Ampullariidce are not unlike those of the Viviparidce, but are mostly larger, and 

 rather more globose. They are covered with a shiny, greenish, or olive 

 periostracum, and often ornamented with transverse colour - bands. All are 

 provided with large ovate opercula of concentric growth, which in species from 

 the Western Hemisphere are thin and horny, whereas in the Old World forms 

 they are thickened internally with a shelly 

 layer. A'lthough these shells have entire 

 mouths (holostomatous), without a canal or 

 even a notch in the aperture, the animal is 

 provided with a long respiratory siphon. 

 The species of Ampullaria are amphibious, 

 and inhabit marshes in tropical countries. 



mi i 1 ; i i J.T i 1 ROW OF TEETH FROM THE EADULA OP 



They are provided with both lungs and (greatly magnified). 



gills, and breathe both air and water. 



Professor Semper observes that these molluscs " breathe not only with both gills and 

 lungs, but they do so in regular alternation ; for a certain time they inhale air at the 

 surface of the water, forming a hollow elongated tube by incurving the margin of 

 the mantle, so that the hollow surface is closed against the water and open only at 

 the top. When they have thus sucked in a sufficient quantity of air, they reverse 

 the margin of the mantle, opening the tube, into which the water streams." 

 They are capable of living out of the water for a long time, and it has been 

 stated that some specimens kept in Calcutta for five years were alive at the end 

 of that period. South America produces the largest forms, but there are also a 

 few handsome species from Central Africa, Madagascar, India, and the Eastern 

 Archipelago. In the genus Lanistes, which occurs only in Africa, the shells are 

 all sinistral, and have horny opercula. 



The families Cydophoridce and Cydostomatidce comprise a large number of 

 air-breathing land-snails, formerly classed with the Pulmonata. The breathing- 

 organ is not a true lung, like that of the snails and slugs, but a vascular branchial 

 chamber, modified for air-breathing, and open in front, the mantle being free 

 above the nape of the neck. The animals are unisexual, and formed much after 

 the fashion of periwinkles. They have a long rostrum, two contractile tentacles, 

 with the eyes at the base, and the radula has seven rows of teeth, arranged as in 

 Littorina and allied genera. Another distinguishing feature is the presence of 

 an operculum, which is possessed by all the species. In the Cydophoridce this is 

 generally horny, circular, and multispiral, with a central nucleus, whereas in the 

 Cydostomatidce it is mostly of a shelly texture, and paucispiral. In the latter 

 family the animals have the sole of the foot divided down the middle by a groove, 

 and, when walking, the halves are alternately advanced. The species of these 

 families are numerous, and are principally found in hot climates. A few, however, 

 occur in more temperate regions ; two, belonging to distinct genera, being found in 

 Britain. The shells are variable in form, and can only be appreciated by studying 

 a series of specimens or figures. The species are classified in a large number of 

 genera, which, although based on a combination of characters, are mostly dis- 

 tinguishable by differences in the opercula. Cyclostoma, Otopoma, Chondropoma, 



