. Nature-study; a manual for teachers and students. Nature study. SOME LOWER ANIMALS 253. backwards and sideways as well as forward. Vast num- bers of lobsters and crabs are caught and eaten on the sea- coasts. MoUusks constitute another type of animal life. Clams, oysters, and snails all are interesting creatures to study. These animals as a rule have a limy shell which is se- creted by the skin. Snails are to be found in ponds and streams, on sticks and stones, and on water plants. Some kinds are found on land, in gar- dens, on bark, and old logs. Some snails have a shell fig. 96. Land Snaii
. Nature-study; a manual for teachers and students. Nature study. SOME LOWER ANIMALS 253. backwards and sideways as well as forward. Vast num- bers of lobsters and crabs are caught and eaten on the sea- coasts. MoUusks constitute another type of animal life. Clams, oysters, and snails all are interesting creatures to study. These animals as a rule have a limy shell which is se- creted by the skin. Snails are to be found in ponds and streams, on sticks and stones, and on water plants. Some kinds are found on land, in gar- dens, on bark, and old logs. Some snails have a shell fig. 96. Land Snaiis. of a cornucopia shape, and others have a f^at spiral. Some, like the slugs, have no regular shell, but only a small, scaly mantle on the back. The shell is increased at the free edge by the addition of new layers. Many snails breathe air in the water by means of gills. Among the aquatic snails the kinds that can close the opening of the shell with a sort of flap or door are gill-breathers. Other aquatic snails are lung-breathers, and must come to the surface of the water periodically for more air. Land snails ako have simple lungs. When a snail is "out of its shell" there will be seen a long,. 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 Holtz, Frederick Leopold, 1870-. New York, C. Scribner's Sons
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