LUNG-BREATHING GASTROPODS 429 



forming the head, with its two pairs of feelers, is indistinctly separated 

 from the elongated foot. The latter supports the shell, which is yellowish- 

 brown, with darker stripes, and encloses the body with the viscera. 

 Along the edge of the shell a lighter-coloured yellowish membrane, the 

 edge of the mantle, is visible. On its right side an opening appears and 

 disappears periodically ; this is the respiratory aperture, which leads into 

 the mantle cavity, or respiratory chamber. 



B. A Land Animal. 



1. The Eoman snail being a land animal, its respiration is effected by 

 a lung, and the respiratory cavity is therefore a pulmonary chamber, the 

 walls of which are traversed by a network of fine bloodvessels, through 

 the thin walls of which the exchange of the respiratory gases is effected 

 (see Part I., p. 6). 



2. Protection against Loss of Moisture. — By reason of their naked, 

 soft body, molluscs are conditioned for an aquatic life. The snail, 

 therefore, being terrestrial in its habits, must, like land amphibians, be 

 protected against the desiccating effect of the air and the parching action 

 of the sun's rays (see Part II., p. 253). Accordingly, as in the last- 

 mentioned group, 



(1) The skin is covered with a viscid mucus, which retards evapora- 

 tion (see also Section C, 1). 



(2) Like them also, the snail is restricted to a moisture-laden atmo- 

 sphere, and is only active on dewy nights, and on days when the air is 

 saturated with aqueous vapour and the soil and plants are moist with 

 rain. The vivifying effects of moisture on this and all other species of 

 land snails may be shown by the following experiment : Put some snails 

 in a vessel in a dry room, and notice that they at once withdraw into 

 their shells, and remain for days, or even weeks and months, in this 

 condition without giving a sign of life. If, however, we pour a little 

 water into the vessel, so that the contained air soon becomes saturated 

 with moisture, or if we sprinkle some of it over the animals, they will 

 soon begin to stir, and in about an hour will be creeping about in the 

 vessel. 



(3) In dry weather and burning sunshine, the snail, like land am- 

 phibians, rests in a safe hiding-place, with this difference, that the snail 

 builds the latter itself, and carries it about with it. During a prolonged 

 drought the water of the mucus secreted by the edge of the mantle 

 evaporates, and a membrane of papery thinness is formed (epiphragm), 

 which still further protects the animal against an excessive loss of water 

 (see also Section C, 5). 



(a) Shape and Size of Shell. — If with a fretsaw we divide a snail 



