XXxiv INTRODUCTION. [CH. 



cially in the winter when there is a deficiency of food. 

 For this reason it may be supposed that all the Mol- 

 Insca hibernate ; and we know that the land- snails in 

 this country have such a habit. Most of them bury 

 themselves in the ground^ or nestle in the crevices of 

 rocks, under the bark of trees, or even in the hollow 

 stems of the larger umbelliferous plants. They also 

 cover the mouths of their shells with a calcareous plate 

 of various degrees of thickness, which they secrete, in the 

 same way as the shells, by means of their mantle. This 

 plate is called an " epiphrag-m,^' and in the apple-snail 

 [Helisc pomatia) is of considerable thickness. But in 

 dry weather and during the heat of summer they form 

 another and slighter kind of epiphragm, in order to keep 

 their bodies always moist and lubricated, as without such 

 protection thfe tissues would soon dry up and the snails 

 perish. The E,ev. H. B. Tristram, in his account of the 

 Great Sahara, says that the snail-shells which he found 

 there were much thicker than those of the same species 

 from more temperate parts of Europe, apparently as an 

 additional means of preventing evaporation in so dry a 

 climate. The simile in the 58th Psalm (verse 8) which 

 is rendered in our translation for the ^ Common Praver/ 

 " consume away like a snail,^^ may have had reference 

 to the inability of these Mollusca to endure exposure 

 to the great heat of the sun in an Eastern climate. 

 None of the naked Slugs occur in the lists of land 

 Mollusca collected by Professor Roth in Palestine, 

 and by Dr. Schliifli and M. Mousson in the East. 

 The circulation of land-snails is affected to a great 

 extent by the temperature. In some kinds the rate 

 of pulsation varies from 30 to 110 per minute during 

 summer ; and it ceases altogether in winter. Although 

 the temperature of the sea is nearly the same in summer 



