II EFFECTS OF COLD AND HEAT 2/ 



mud in deeper water.^ Limnaea and Planorhis have often been 

 noticed to crawl about under the lower surface of a thick coat- 

 ing of ice. In periods of prolonged drought, when the water in 

 the ponds dries up, the majority of genera bury themselves in 

 the mud. I have known Limnaea peregra bury itself three 

 inches deep, when surprised by a sudden fall of the water in 

 the ditch on Coe Fen, behind Peterhouse, Cambridge. Physa 

 hypnorum frequents by preference ditches which dry up in sum- 

 mer, as does also Planorhis spirorhis^ the latter often forming a 

 sort of epiphragm against evaporation. Aiicylus has been ob- 

 served to spend the whole winter out of water, and P. spirorbis 

 has been noticed alive after four months' desiccation.^ 



True aestivation, however, occurs mainly in the tropics, 

 where there is no winter, but only a period when it is not quite 

 so hot as the rest of the year, or on a coast like the Mediter- 

 ranean, which is subject to sudden and severe heat. This period 

 is usually rainless, and the heat is therefore a dry heat. At this 

 season, which may last for three or four months, most of the 

 land MoUusca enter upon a period of inaction, either burying 

 themselves deeply in the ground, or else permanently attaching 

 themselves to the stalks of grass and other herbage, or the under 

 sides of rocks. For instance, the large and beautifully painted 

 Orthalicus^ Corona, 'dud Poyyhyrohaphe, which inhabit Brazil, 

 Ecuador, and eastern Peru, bury themselves deeply in the 

 ground during the dry season, while in the rains they climb to 

 the topmost branches of the great forest trees.^ Thus it may 

 well happen that a visitor to a tropical island, Ceylon for in- 

 stance, or one of the Greater Antilles, if he time his visit to 

 coincide with the rainless season, may be grievously disap- 

 pointed at what seems its unaccountable poverty in land Mol- 

 lusca. But as soon as the weather breaks, and the moisture 

 penetrates their retreats, every bush and every stone, in favoured 

 localities, will be alive with interesting species. 



The Epiphragm. — A considerable number of the land Pul- 

 monata (and a very few of the fresh-water) possess the power 

 of closing the aperture of their shell by means of what is known 

 as an epiphragm or covering of hardened mucus. This epi- 

 phragm is habitually formed by certain species during hiberna- 



1 Reichel, Zool. Anz. x. p. 488. 2 Schumann, Schr. Ges. Danz. (2) vi. p. 159. 

 3 Fischer and Crosse, Mexico^ p. 437. 



