GASTROPODS. 
389 
BOW OF TEETH FROM THE RADULA OF Vivipam 
(greatly magnified). 
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. Although 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. 
They are provided with both lungs and 
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.” 
Thejr are capable of living out of the water for a long time, and it has been 
stated that some specimens kept in Calcutta for live 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 Cyclop!loridce and Cyclostomatidce 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. I11 the Cyclop)\oridce this is 
generally horny, circular, and multispiral, with a central nucleus, whereas in the 
Cyclostomatidce 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, 
