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OF SHELLS. 87 |
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gradually increased by the supply of a viscous substance, exuding from the animal, until it is furnished with a dwelling completely adapted to its wants and situation. It is very interesting to notice the great variety of habitations provided for the different species, and to see how admirably each one is in accordance with the instincts of the occupant. Some are sheltered by thick coverings, which protect them from the beating of the boisterous waves; others, which inhabit the still waters of ponds or ditches, have light and fragile dwellings. The shells of some animals are in form like little vessels, the valves of which they open to the breeze, and thus float on the surface of the waves; others, in- |
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