. Outlines of zoology. phridia. The rest of the cavity of the body is vascular system is almost always well developed, butpart of the circulation is in most cases lacunar; the hearttypically consists of a ventricle and two auricles. Respiratoryorgans are most typically represented by gills or ctenidia,consisting of an axis attached to the body and bearing lamellcR,hut the gills may have simpler forms, or may be absent, andin the terrestrial snails the mantle cavity is adapted foraerial respiration. At the base of the gills there is getierallyan olfactory organ or vsphradium. The
. Outlines of zoology. phridia. The rest of the cavity of the body is vascular system is almost always well developed, butpart of the circulation is in most cases lacunar; the hearttypically consists of a ventricle and two auricles. Respiratoryorgans are most typically represented by gills or ctenidia,consisting of an axis attached to the body and bearing lamellcR,hut the gills may have simpler forms, or may be absent, andin the terrestrial snails the mantle cavity is adapted foraerial respiration. At the base of the gills there is getierallyan olfactory organ or vsphradium. The sexes are separateor united. There are two common larval stages, — theTrochosphere, which resembles the same stage in someAnnelids, and the more characteristic Veliger (Fig. 206) ;but the development is often direct. First Type of Mollusca. The Snail (^Helix), one of theterrestrial (pulmonate) Gasteropods Habits.—The common garden snail {H. aspersa), or thelarger edible snail {H. pomatia), which is rare in England. y §^P Fig. 207.—Roman snail {Helixpomatia). Note shell covering visceral hump ; pulmonary apertuie(including anus and opening of ureter) ;/., the ioo\.\ aperture; m., mouth ; e.^ eye on long horn ; , one ofshort horns. but abundant on the Continent, serves as a convenient typeof this large genus of land-snails. They are thoroughly THE SNAIL. 383 terrestrial animals, breathing air directly through a pulmon-ary chamber, and drowning (slowly) when immersed inwater. Their food consists of leaves and other parts ofplants, but they sometimes indulge in strange vagaries ofappetite. They are hermaphrodite, but there is alwayscross-fertilisation. The breeding time is spring, and theeggs are laid in the ground. In winter snails bury them-selves, usually in companies, cement the mouths of theirshells with hardened mucus and a little lime, and fall into astate of latent life, in which the heart beats feebly. Theyhave been known to remain do
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
