. Animal biology. Biology; Zoology; Physiology. SURVEY OF INVERTEBRATES 87 for locomotion; and a mantle cavity between the main body and an enclosing envelope, the mantle. Among the five classes of Molluscs only three are of sufficient general interest to command our attention here: the Gastropoda, Pelecypoda, and Cephalopoda. Gastropoda. One usually thinks of Molluscs as sea-dwelling animals but among the some sixteen thousand species of Snails, Slugs, and other similar Gastropods, more than one-third are terrestrial. Everyone is familiar with the spirally-coiled Snail's shell into which the
. Animal biology. Biology; Zoology; Physiology. SURVEY OF INVERTEBRATES 87 for locomotion; and a mantle cavity between the main body and an enclosing envelope, the mantle. Among the five classes of Molluscs only three are of sufficient general interest to command our attention here: the Gastropoda, Pelecypoda, and Cephalopoda. Gastropoda. One usually thinks of Molluscs as sea-dwelling animals but among the some sixteen thousand species of Snails, Slugs, and other similar Gastropods, more than one-third are terrestrial. Everyone is familiar with the spirally-coiled Snail's shell into which the animal can completely retire when disturbed, but many close relatives, such as the Limpets, have shells that are merely simple flattened cones, while most of the Slugs have no shell at all. The shell when present is secreted chiefly by the in-. Intestine, Shell Mantle cavity Mantle. 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 Woodruff, Lorande Loss, 1879-1947. New York The Macmillan company
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