. Outlines of zoology. Zoology. 364 MOLLUSC A. Best developed in Gasteropods and Cephalopods, the head region may elsewhere be represented, as in Dentalium, merely by a buccal tube fringed with tentacles. Apart from Lamellibranchs, the radula is characteristic and, with few exceptions, universal. Almost as important is the condition of the characteristic Molluscan foot. Primitively this had the form of a ventral creeping sole, as shown, for example, in its simplest condition, in Chiton (Fig. 165). This condition is retained in many Gasteropods, and in the simplest Lamellibranchs, like Sole- no
. Outlines of zoology. Zoology. 364 MOLLUSC A. Best developed in Gasteropods and Cephalopods, the head region may elsewhere be represented, as in Dentalium, merely by a buccal tube fringed with tentacles. Apart from Lamellibranchs, the radula is characteristic and, with few exceptions, universal. Almost as important is the condition of the characteristic Molluscan foot. Primitively this had the form of a ventral creeping sole, as shown, for example, in its simplest condition, in Chiton (Fig. 165). This condition is retained in many Gasteropods, and in the simplest Lamellibranchs, like Sole- nomya. In most Lamellibranchs, however, in adaptation to a more or less passive life in the sand, the foot became wedge-. Fig. 158.—Common Buckie (Buccinmn undatvm). Eye ; o., respiratory siphon ; o., operculum ;/., foot. shaped, and the characteristic byssus gland, which secretes attaching threads, is developed. In the active cuttles the foot became greatly modified, and in those related to Sepia a portion of it is specialised as the funnel—the main organ of active locomotion. That the condition of the foot cannot in itself be employed as a basis of classification, is, however, obvious, when its differences within the limits of a class are considered. Thus it is obsolete in the pelagic Phyllirho'e among Gasteropods, in the sedentary oyster among Lamelli- branchs ; in the pelagic Pteropods part of it forms lateral wing-like lobes used in swimming, while in lanthina, which has a similar habit, its chief use is to secrete a " float" to which the egg-capsules are attached. In various Lamelli-. 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
