. Outlines of zoology. That thecondition of the foot cannot in itself be employed as a basisof classification is, however, obvious, when its differenceswithin the limits of a class are considered. Thus it isobsolete in the pelagic Phyllirhoe among Gasteropods, in GENERAL NOTES ON MOLLUSCS. 413 the sedentary oyster among Lamellibranchs; in the pelagicPteropods part of it forms lateral wing-like lobes used inswimming, while in lanthina, which has a similar habit, itschief use is to secrete a float to which the egg-capsulesare attached. In various fjamellibranchs, and in Dentalium,it is modified


. Outlines of zoology. That thecondition of the foot cannot in itself be employed as a basisof classification is, however, obvious, when its differenceswithin the limits of a class are considered. Thus it isobsolete in the pelagic Phyllirhoe among Gasteropods, in GENERAL NOTES ON MOLLUSCS. 413 the sedentary oyster among Lamellibranchs; in the pelagicPteropods part of it forms lateral wing-like lobes used inswimming, while in lanthina, which has a similar habit, itschief use is to secrete a float to which the egg-capsulesare attached. In various fjamellibranchs, and in Dentalium,it is modified as a conical boring organ. The mantle is another important MoUuscan structure,and as it secretes the shell, the shape of the latter is ofcourse determined by it. Primitively the mantle is repre-sented by a uniform downgrowth of skin from the dorsalsurface, surrounding the ventral foot, and secreting a dorsalcap-shaped shell. Such a simple condition occurs in thelimpet. In the Lamellibranchs, with the lateral flattening. Fig. 223.—Bivalve (Panopcea norvegica), showing , Exhalant aperture ; i., inhalant aperture. of the body, the mantle becomes divided into right and lefthalves, and the shell becomes two-valved. In most Lamelli-branchs the mantle is prolonged into two tubes or siphons,through which the water of respiration enters and leaves themantle cavity. A similar but unpaired siphon is found inmany Gasteropods. In Scaphopoda the mantle folds fuseventrally to form a continuous tube. In most Gasteropodsthe. mantle skirt is retained, and secretes a spiral shell, aswell as enclosing a space in which the gills lie; insome, both mantle and shell are absent. In the snailand its allies (Pulmonata), the mantle forms thepulmonary chamber, which opens to the exterior by asmall aperture. In Cephalopoda the mantle skirt is welldeveloped and muscular, and, besides sheltering the gills, isof much importance in locomotion. 414 PHYLUM MOLLUSC A. Typically the Mollusca are bilater


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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology