. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. MOLLUSCA ACEPHALA. 195 generally more or less of a spiral form, and are then called turbmated shells. In a few, the cavity of the shell is divided by transverse partitions into numerous compartments. Some MoUusca have in- ternal shells for the defence and support of par- ticular organs; and others have shells which are partly external, and partly internal. As respects their shape, colour, and appearance, shells admit of infinite diversity ; yet, as will pres
. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. MOLLUSCA ACEPHALA. 195 generally more or less of a spiral form, and are then called turbmated shells. In a few, the cavity of the shell is divided by transverse partitions into numerous compartments. Some MoUusca have in- ternal shells for the defence and support of par- ticular organs; and others have shells which are partly external, and partly internal. As respects their shape, colour, and appearance, shells admit of infinite diversity ; yet, as will presently be shown, all are composed of the same kind of material; and their production and increase are regulated by the same uniform laws. § 2. Acephala. The Mollusca which inhabit bivalve shells, such as the Oyster, the Muscle, and the Cockle, are all acephalous. The two valves of the shell are united at the back by a hinge joint, often very artificially constructed, having teeth that lock into each other: and the mechanism of this articulation varies much in different species. The hinge is secured by a sub- stance of great strength. It is seen in Fig. 101, which shows the valves of the Unio batava, with the connecting ligament. This ligament is com- posed of two kinds of texture : the one, which is always external, is strictlyligamentous; that is, perfectly inelastic: the other has more of the. 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 Roget, Peter Mark, 1779-1869. London : W. Pickering
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