The soft slime which is yielded by the body of the animal, _hardens upon the orifice of the shell_, and thus increases its size. [Illustration: Fig. 80.--COMMON GARDEN SNAIL.] 1308. _Why is the shell spiral?_ Partly because of its original formation; but also because, _as the shell grows_, the opening is elongated; and thrown up, causing the spiral body of the shell to turn, and so to wind its growth around the centre. [Verse: "Notwithstanding they hearkened not unto Moses; but some of them left it until the morning, and it bred worms, and stank: and Moses was wrath with them."--EXODUS XVI.] 1309. _Why has the snail four tentacula attached to its head?_ Because the insect, having no other limbs, is provided with those projecting members, the lower two serving as _feelers_ and the upper two also as _feelers_ and _eyes_. These, projecting in the front of the animal, impart to it a consciousness of surrounding objects, and especially of those which lie in its path. 1310. _Why is the snail able to move, without feet?_ Because it has attached to its body a fringe of muscular skin, which is capable of considerable contraction and expansion, and by alternately stretching and shortening this, the snail is able to draw himself along. 1311. _Why do we see no snails in the winter time?_ Because they bury themselves in the ground, or in holes, where they remain _in a torpid state_ for several months. Before they enter into the torpid state, they form with their slimy secretion, and with some earthy matters which they collect, a strong cement with which they seal up the opening to their shells. 1312. _Why can snails live in shells thus sealed?_ Because they leave, in the thin wall by which they close themselves in, _a small hole_, too small to admit water, but large enough to let in sufficient air to carry on their feeble respiration during their winter sleep. 1313. _Why do insects abound in putrid waters, and in decaying substances?_ Because they have been endowed with appetites and with constitutions that enable them to live upon and to enjoy corrupt matter. In this point of view the maggots of flies are exceedingly useful; a dead carcass is speedily threaded by them in every direction; thus that corrupt matter which, in a large mass, would poison the air, is taken up in small portions by millions of living bodies, and by them _dispersed_, and becomes innoxious. [Verse: "For he maketh small the drops of water: they pour down rain according to the vapour thereof."--JOB XXXV.] 1314. _Why do we see, in tanks of rain water, insects rising to the surface?_ Because numerous insects pass through their first stages of existence in _water_, and among them the common gnat. The gnats of the previous season having deposited their eggs on the sides of the water-butt, the warm water developes them, and the larvæ of the gnats appear (Fig. 81; _c_ natural size of larva; _b_ larva magnified). [Illustration: Fig. 81.--LARVA AND PUPA OF GNAT.