. Elementary text-book of zoology [electronic resource]. Zoology. 284 MOLLUSC A. comprises the snails and the slugs. 1 he rest are marine or freshwater. Some, the sea-slugs or JVudibranchs, lose their shells and have an external approximation to piano- symmetry. Others are adapted for a pelagic life, they are usually transparent, and the shells if present are thin and pellucid. The foot is usually reduced, but may form a swimming organ. The great majority of the sub-class, ho^â ever, creep on the sea-floor, and may be carnivorous scavengers, e.g., whelks, or herbivorous, e.g., periwinkles. Th
![. Elementary text-book of zoology [electronic resource]. Zoology. 284 MOLLUSC A. comprises the snails and the slugs. 1 he rest are marine or freshwater. Some, the sea-slugs or JVudibranchs, lose their shells and have an external approximation to piano- symmetry. Others are adapted for a pelagic life, they are usually transparent, and the shells if present are thin and pellucid. The foot is usually reduced, but may form a swimming organ. The great majority of the sub-class, ho^â ever, creep on the sea-floor, and may be carnivorous scavengers, e.g., whelks, or herbivorous, e.g., periwinkles. Th Stock Photo](https://c8.alamy.com/comp/RJNAMN/elementary-text-book-of-zoology-electronic-resource-zoology-284-mollusc-a-comprises-the-snails-and-the-slugs-1-he-rest-are-marine-or-freshwater-some-the-sea-slugs-or-jvudibranchs-lose-their-shells-and-have-an-external-approximation-to-piano-symmetry-others-are-adapted-for-a-pelagic-life-they-are-usually-transparent-and-the-shells-if-present-are-thin-and-pellucid-the-foot-is-usually-reduced-but-may-form-a-swimming-organ-the-great-majority-of-the-sub-class-ho-ever-creep-on-the-sea-floor-and-may-be-carnivorous-scavengers-eg-whelks-or-herbivorous-eg-periwinkles-th-RJNAMN.jpg)
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. Elementary text-book of zoology [electronic resource]. Zoology. 284 MOLLUSC A. comprises the snails and the slugs. 1 he rest are marine or freshwater. Some, the sea-slugs or JVudibranchs, lose their shells and have an external approximation to piano- symmetry. Others are adapted for a pelagic life, they are usually transparent, and the shells if present are thin and pellucid. The foot is usually reduced, but may form a swimming organ. The great majority of the sub-class, ho^â ever, creep on the sea-floor, and may be carnivorous scavengers, e.g., whelks, or herbivorous, e.g., periwinkles. The shells of such types as the limpets and earshells {Haliotis) are not spirally twisted. Fig. 198.âA Belermite Restored. (After Owen.) Class II.âLamellibranchiata. The bivalve Mollusca are usually completely enveloped in the paired shells. I'he ctenidia have been enormously developed, and serve to feed the animal. They are mostly burrowing types, all aquatic, and most are marine. They illustrate degrees in degeneration, the oyster entirely losing its foot. The scallop iyPecten) moves actively through the water by snapping its shells together. Teredo is a worm-like form with very small shells, which bores its Nay through wood. Cockles and mussels are other common species. Class III.âCephalopoda. In these the molluscan plan reaches its highest level. Sepia is a very fair type of the class. They are all active free- swimming forms, with the fore part of the foot produced into tentacles, the hind part into a syphon, and the organs are piano-symmetric.. g. Eight hooked tentacles (the other two longer tentacles are not shown); f, fins ; e, posterior part containing the shell ; b, phragmacone; d, ink-sac; z, syphon (or funnel).. 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 work.. Masterman, Arthur Thomas; Parsons,