. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. MOLLUSKS 267 plants on the surface of the aquarium and the action of the hngual ribbon noted. Breathing. — Most gastropods breathe; by means of gills which are lo- cated, as in the clam, in the mantle cavity. But in some snails, the pulmonates, the mantle cavity forms a sac which opens to the outside of the body by a tiny slit. In this cavity air is taken. Its walls are filled with blood vessels, and oxygen is then taken up by the blood as carbon dioxide is released. The pulmonates are true
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. MOLLUSKS 267 plants on the surface of the aquarium and the action of the hngual ribbon noted. Breathing. — Most gastropods breathe; by means of gills which are lo- cated, as in the clam, in the mantle cavity. But in some snails, the pulmonates, the mantle cavity forms a sac which opens to the outside of the body by a tiny slit. In this cavity air is taken. Its walls are filled with blood vessels, and oxygen is then taken up by the blood as carbon dioxide is released. The pulmonates are true air breathers, and may frequently be seen in an aquarium taking down a bubble of air for use under water. Senses. — Snails and slugs can distinguish light from darkness, as may be easily proved by experiment. The tentacles are the most sensitive parts of the body. Certain parts of the animal seem to be used for testing the water, and in the land snails these organs seem to be used to smell with. The nervous system as in the clam consists of three collections of nerve cells, or ganglia, nerves connecting these centers, and numerous cells in the outer part of the body called sensory cells. These cells are sensitive to any stimulus received from outside the body. Development. — Eggs of the pond snails are laid in little masses, sometimes in strings, and are often found fastened in a jelly to the side of the aquarium. The snails hatch and at first swim about, later settling down as the shell is formed. Thus they undergo a complete metamorphosis, as do their relatives the clams and oysters. Cephalopods. — Squid, Cuttlefish, and Octopus. The name cephalopod means head-footed. As the figure shows, the mouth is surrounded with a circle of ten- tacles. The shell is internal or lacking, the so-called pen of the cuttlefish being all that remains of the shell. The squid, or cuttlefish, is strangely modified for the hfe it leads. It moves through the water more swdftly than a fish
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