True Snails (_Helicidæ_, etc.) have a distinct head furnished with eyes, tentacles, cutting upper jaws, and rasping teeth, and all are protected by a spiral shell. They are almost exclusively vegetable-feeders, subsisting chiefly on leaves. The sexes are not distinct. Many of the species are beautiful objects on account of the brilliancy of their colouration, and some are remarkable for the variation they exhibit in this respect. Species of _Helicidæ_ are found in nearly every part of the world and in all situations, from sea-level to an altitude of 12,000 feet. They are fond of moisture, and in hot and dry weather retire within their shells, remaining torpid until the return of dew and rain. _Helix pomatia_ (Case 119), which is found on the chalk in the south of England and on the Continent, is commonly eaten in Austria, France, and Belgium. The eggs of Land-Snails vary in texture, size, and in numbers: they are usually white, but in some instances yellow and pale green. Those of some of the large South-American forms are as hard as that of a hen, and more than an inch in length (Case 120). [Illustration: Fig. 23. British Land-Snail (_Helix pomatia_). _a_, eye-bearing tentacles (“horns”); _b_, lower or smaller tentacles. ] Slugs (Cases 106,107) are very like Snails without external shells; most of them, however, possess a small internal shelly plate, or a few calcareous granules hidden beneath the skin of the back. Some have a large slime-pore at the end of the foot, and others are slightly phosphorescent. Like the Snails, they are fond of damp localities, and at times become great pests to farmers in devouring the young shoots of the growing corn. _Testacella_, which is found in this country, differs from the Slugs in having an external shell at the tail-end of the foot. It is not slimy, and lives under ground, feeding upon earthworms. Class III.—SCAPHOPODA.[5] [Illustration: Fig. 24. British Tooth-shell. (_Dentalium tarentinum_). _a._ The shell. _b._ The animal, removed from its shell; _f._ the foot. ] [Sidenote: Case 136.] The “Tooth-shells” (_Dentaliidæ_) form a distinct group, the shells of which are very unlike those of any other mollusc, but closely resembling the shelly tubes constructed by certain kinds of marine worms. The _Dentalia_ have neither eyes nor tentacles, or a distinct head like Gastropods; their organs of circulation and respiration are of a rudimentary kind, and they have no heart. The sexes are separate. Their foot is adapted for burrowing in sand, in which they live and obtain their food, which consists of _Foraminifera_ and minute Bivalves. One species, _Dentalium pretiosum_, found on the shores of North-West America, was until recently used as money by the Indians. Class IV.—LAMELLIBRANCHIA.[6] [Sidenote: Cases 137–204.]