Page:Popular Science Monthly Volume 45.djvu/113

Rh The ampullarias or idol shells, a noble genus of tropical pond snails, bury themselves deeply in mud during the dry seasons. They are remarkable for their ability to live without water, having been kept out of it for years, and they are often brought to foreign countries alive in mahogany logs.

Guilding first noticed that the species of the Antilles had a double system of respiration, which was further dilated on by Caillaud, who brought these snails alive from Egypt; and D'Orbigny discovered that they had a distinct pulmonary apparatus in addition to their gills. According to Joly, anodons and viviparas survive freezing, and will reproduce on being thawed out; and no doubt many of the species that live in cold climates are frozen every winter and resuscitated with the return of spring.

It is believed that all shell-bearing land mollusks either hibernate or æstivate according to conditions of climate. Most of the snails close the aperture with a membranous or coriaceous covering, consisting of lime and mucus, which is called an epiphragm. W. G. Binney has thus described the operation: "The animal being withdrawn into the shell, the collar is brought to a level with the aperture and a quantity of mucus is poured out and covers it. A small quantity of air is then emitted from the respiratory foramen, which detaches the mucus from the surface of the collar and projects it in a convex form like a bubble. At the same moment the animal retreats farther into the shell, leaving a vacuum between itself and the membrane, which is consequently pressed back by the external air to a level with the aperture or even farther, so as to form a concave surface, where, after becoming desiccated and hard, it remains fixed. These operations are nearly simultaneous, and occupy but an instant." As the winter advances the snail withdraws deeper and deeper, shutting itself out by other epiphragms, like a retiring army covering its front by breastworks as it retreats, until sometimes it has made no less than half a dozen, one within the other. With the snails such as ours, that inhabit moist wooded districts, this protecting wall is thin and nearly transparent, while in those of arid regions it is thicker and often calcareous. Some of the large helices of south Europe secrete a somewhat shelly epiphragm resembling the coating of a turtle's egg, convex externally, with the edge turned in and roughly cemented to the aperture of the shell. In this condition, if not resuscitated by moisture, the snails will remain alive for an indefinite period. Woodward tells of a desert snail (