XXXIV 



INTRODUCTION. [CH. 



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



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



lusca 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 " epiphragm,^' and in the apple-snail 



{Helix 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 th6 tissues would soon diy up and the snails 



perish. The Rev. 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 diy a 



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



is rendered in our translation for the ' Common Prayer/ 



" 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. Schlafli and M. Mousson in the East. 



The circulation of land-snails is aflPected 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 



