ROW OF TEETH FROM THE RADUtA OF 



FRONT-GILLED GROUP 3359 



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 color bands. All are provided 

 with large ovate opercula or 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. Although 

 these shells have entire mouths (holostom- 

 atous), 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. They are 



11 -,11,11 j -11 j now *JJP i. ivrvi Jti rtujivi m!, K.ALHJ L.J 



provided with both lungs and gills, and 17 . . 



Vivipara (greatly magnified), 

 breathe both air and water. Professor 



Semper observes that these mollusks " breathe not only with both gills and lungs, 

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

 face 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 cham- 

 ber, 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 Cyclophoridce 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 princi- 

 pally 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 characteristics, are mostly distinguishable by differences in the oper- 

 cula. Cydostoma, Otopoma, Chondropoma, Choanopoma, Cistula, Tudora, Omphalo- 



