. Outlines of zoology. 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 Mollus
. Outlines of zoology. 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 bilaterally symmetricalanimals, and this symmetry is marked in the Solenogastres,the Lamellibranchiata, and occurs to a less extent in theCephalopoda (cf. the unpaired genital organs). In mostGasteropoda it is completely lost. This seems to be insome way associated with the dorsal displacement of theviscera in Gasteropods to form the (usually coiled) visceralhump. In Cephalopods there is a somewhat similar dis-. ^^fe Fig. 224.—Nudibranch [Dendronottts arborescens), showingdorsal oufgrowths forming adaptive gills. placement in a postero-dorsal direction, in Lamellibranchsin a ventral direction, but in neither case is it so marked asin Gasteropods. The characters of the internal organs of Mollusca mustbe inferred from the description of the types, but the natureof the respiratory organs may be briefly noted. Typically,these consist of two feathery gills, or ctenidia, with an axisattached to the body and bearing a double row of lamellae!These are sheltered beneath the mantle, and bear at theirbases two osphradia or smelling patches. Gills of thistypical form occur in Cuttles {Nautilus has four), in thesimplest Gasteropods (but many other Gasteropods have asimple unpaired gill), and in the lowest Lamellibranchs{Solenomya, Nucula, etc.). The respiratory organs in otherMollusca show much diversity when compared with thisprimitive type. Thus the gills may be totally suppresse
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
