Читать книгу The Book of Shells. Containing the Classes Mollusca, Conchifera, Cirrhipeda, Annulata, and Crustacea онлайн

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The organs of motion in the Mollusca, according to their different form and position, give names to most of the orders; these consist of muscular expansions of the body, by means of which the animal swims or drags itself along the ground. The gills, or breathing apparatus, are situated internally, and communicate with the air or water, by means of a small canal opening outwardly. The mouth is usually concealed from view when the creature is at rest; in some, this organ is furnished with a hard substance, which supplies the place of teeth, while, in others, it is in the form of a projecting tube. The greatest portion of these creatures are produced in the water, the tribes that inhabit that element, exceeding by far those that are to be found upon the land.

The Molluscous animals have been separated into the five following Orders:—

 1. Heteropoda, (with feet, or organs of motion,

 not uniform in all species.)

 2. Cephalopoda, (with feet, or organs of motion,

 attached to the head.)

 3. Trachelipoda, (with feet, or organs of motion,