. An introduction to zoology, with directions for practical work (invertebrates). MOLLUSC A 119 the animal withdraws into its shell, just fits into and closes the mouth of the shell. This operculum is formed as a cuticular structure by the secretion of a group of cells on the dorsal side of the foot, at its posterior end. Genus Fresli-water Winkles.—These snails are vivi- Faludiua. parous, the eggs are hatched within the body of the mother; the fry are not set free until they are at least two months old, when they are gradually ejected, two or three at a time. The two British species of t
. An introduction to zoology, with directions for practical work (invertebrates). MOLLUSC A 119 the animal withdraws into its shell, just fits into and closes the mouth of the shell. This operculum is formed as a cuticular structure by the secretion of a group of cells on the dorsal side of the foot, at its posterior end. Genus Fresli-water Winkles.—These snails are vivi- Faludiua. parous, the eggs are hatched within the body of the mother; the fry are not set free until they are at least two months old, when they are gradually ejected, two or three at a time. The two British species of this genus are both large snails having a shell of 6 or 7 whorls with very convex surfaces; in both of them the foot bears on its upper surface a horny plate, or "operculum," with irregularly con- centric lines of growth on it. When the snail withdraws into its shell, this plate completely closes the aperture. There are two tentacles, with an eye placed on a little tubercle on the outer side of the base of each. The mouth is borne on a prominent snout. Paludina vivipara (see Fig. 71).— This species is fairly common in ponds and slow-flowing streams. The body is dark, speckled with yellow. The shell has a rather thick operculum, and there are three dark bands on the body whorl of the shell, running spirally round it. The form of the teeth on the rasp is shown in Fig. 70. Paludina conteda.—This species is not so common as vivipara, though not rare in the south of England. It is slightly larger than the other species and the shell has a thinner operculum, more swollen whorls, a more pointed spire, and a more distinct umbilicus. It is a very irritable creature, falling with surprising rapidity from. 6 b ^ Fig. 71. —Paludina vivipara. A, Female snail; B and C, males; /, foot; &, breathing aperture; 0, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance
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Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1913
