THE SLEEP OF MOLLUSKS. 101 



The ampullarias or idol shells, a no"ble genus of tropical pond 

 snails, bury themselves deeply in mud during the dry seasons. 

 They are remarkable for their ability to live without water, hav- 

 ing 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'Or- 

 bigny discovered that they had a distinct pulmonary apparatus 

 in addition to their gills. According to Joly, anodons and vivip- 

 aras 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 hiber- 

 nate or sestivate according to conditions of climate. Most of the 

 snails close the aperture with a membranous or coriaceous cover- 

 ing, 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 being withdrawn into the shell, the collar is brought to a level 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 re- 

 spiratory 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 becom- 

 ing 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 (Helix deser- 



Brule, Nebraska, the stream at that time being at an ordinary stage of water. These were 

 kept in a dry shed some two weeks, and the shells became badly cracked by the dry air, yet 

 at the end of that time when opened they were alive. In summer the river becomes so dry 

 that the sand from its bed is blown about the adjoining country. 



