. Outlines of zoology. Zoology. 366 MOLLUSC A. Gasteropoda it is completely lost. This seems to be in some way associated with the dorsal displacement of the. Fig. 160.—Nudibranch (Dendronotus arborescens), showing dorsal outgrowths forming adaptive gills. viscera in Gasteropods to form the (usually coiled) visceral hump. In Cephalopods there is a somewhat similar dis- placement in a postero-dorsal direction, in Lamellibranchs in a ventral direction, but in neither case is it so marked as in Gasteropods. The characters of the inter- nal organs of Mollusca must be gathered from the description
. Outlines of zoology. Zoology. 366 MOLLUSC A. Gasteropoda it is completely lost. This seems to be in some way associated with the dorsal displacement of the. Fig. 160.—Nudibranch (Dendronotus arborescens), showing dorsal outgrowths forming adaptive gills. viscera in Gasteropods to form the (usually coiled) visceral hump. In Cephalopods there is a somewhat similar dis- placement in a postero-dorsal direction, in Lamellibranchs in a ventral direction, but in neither case is it so marked as in Gasteropods. The characters of the inter- nal organs of Mollusca must be gathered from the description of the types, but the nature of the respiratory organs may be briefly noted. Typically, these consist of two feathery gills, sheltered beneath the mantle, and bearing at their bases two Fig. 161.—Ventral surface of osphradia or smelling patches. p„t„iin imlontn.—After Forbes ^,.\. r it . . , r Gills of this typical form occur in Cuttles (Nautilus has four), in foot, and vascular the simplest Gasteropods (but ntle replacing the i /-, i * many other Gasteropods have a simple unpaired gill), and in the lowest Lamellibranchs (Solenomya, Nucula, etc.). The respir- atory organs in other Mollusca show much variation when compared with this primitive type. Thus the gills may be. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. Edinburgh, Glasgow and London, H. Frowde and Hodder & Stoughton
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
