Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 PHYLUM IV. -MOLLUSCA. Oysters, clams, snails, and cuttlefish may be taken as examples of the ten thousand different species which are known as molluscs. The name comes from the Latin mollis, soft, and alludes to the fact that, aside from the shell, the body has no conspicuous hard parts. This, how- ever, is a point of no real importance in classifying animals. Molluscs vary greatly in appearance; but if we carefully compare the points which all possess in common, we can construct an idea
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 PHYLUM IV. -MOLLUSCA. Oysters, clams, snails, and cuttlefish may be taken as examples of the ten thousand different species which are known as molluscs. The name comes from the Latin mollis, soft, and alludes to the fact that, aside from the shell, the body has no conspicuous hard parts. This, how- ever, is a point of no real importance in classifying animals. Molluscs vary greatly in appearance; but if we carefully compare the points which all possess in common, we can construct an ideal mollusc, from which anv form mav be •/ «/ derived by additions here and modifications there. Such a typical mollusc is described below (fig. 34). FIG. 34.—Transverse and longitudinal sections of a schematic Mollusc, a, auri- cle; c, cerebral ganglia; d, digestive tract; /.foot; g, gill; h, heart; i, intes- tine; I, liver m, mouth; n, nervous system; p, pedal ganglia; pc, pericar- dium; s, stomach; v, vent (in left figure, ventricle). The body is saccular, and bilaterally symmetrical. There is, above, a conical visceral mass; below, a muscular foot; while from either side a fold of the body-wall extends outwards and downwards as a mantle. Between the 193
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