THE NATURAL HISTORY SURVEY. 139 



over the aperture. The formation of this membrane (epi- 

 phragm) is as follows: "The animal being withdrawn into the 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 from it 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 mo- 

 ment, 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, having become 

 desiccated and hard, it remains fixed. These operations are 

 nearly simultaneous and occupy but an instant. As the weather 

 becomes colder the animal retires farther into the shell and 

 makes another septum, and so on, until there are sometimes as 

 many as six of these partitions."* 



During hibernation the heart almost ceases to beat, and all 

 the functions of the body cease, the animal becoming torpid, 

 to be awakened only when the warm days of April or May ap- 

 proach. The reason for hibernation is seemingly only to enable 

 the organism to withstand the cold climate, and to exist during 

 a period when the food supply is cut off. When kept in con- 

 finement they do not generally hibernate, but live during the 

 winter as in the summer. 



The comparative speed of snails while in motion does not 

 seem to have been studied to any great extent. As a matter 

 of curiosity the writer timed a few species, with the result as 

 tabulated below: 



Limnaea palustris ........................ 2 inches in 45 seconds. 



SUBORDER STYLOMMATOPHORA 



Tentacles four in number, the superior retractile, with eyes 



*\V. G. Binney, "A Manual of American Land Shells, p. IO-H." Bull. U. S. Nat. 

 Museum, No. 28, 1885. 



