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plants on the surface of the aquarium and the action of the lingual ribbon noted.
BREATHING. Most gastropods breathe by means of gills which are located, 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 smel 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 tentacles. 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 life it leads. It moves through the water more swiftly than a fish by squirting water from the siphon. It can seize its prey with the suckers on the long tentacles and tear it in pieces by means of its horny parrotlike beak. It is protected from its enemies and is enabled to catch its prey because of its ability to change color The squid. One fourth quickly. This change of color is caused by the movement of certain color-bearing cells under the skin. In this way the animal simulates its surroundings. The squid also has an ink bag near the siphon which contains the black sepia. A few drops of this ink squirted into the water effectually hide the animal from its enemy.
natural size. vison, Zoology.
To this group of animals belongs also the octopus, or devil fish, a cephalopod known to have tentacles over thirty feet in length. The paper
nautilus and pearly nautilus, the latter made famous by our poet Holmes, also belong to this group.
Habitat of the Mollusks. Mollusks are found in almost all parts of the earth and sea. They are more abundant in temperate localities than elsewhere, but are found in tropical and arctic countries. They are found in all depths of water, but by far the greatest number of species live in shallow water near the shore. The cephalopods live near the surface of the ocean, where they prey upon small fish. The food supply evidently determines to a large extent where the animal shall live. Some mollusks are scavengers, others feed on living plants.
We have found in the forms of Mollusca studied that almost all mollusks live in the water. There is one great group which forms a general exception to this, certain of the snails and slugs called pulmonates. But even these anin. ls are found in damp localities, and at the approach of drought they become inactive, remaining within the shell. The European snail (Helix pomatia), imported to this country as a table delicacy, exists for months by plugging up the aperture to the shell with a mass of slimy material which later hardens, thus protecting the soft body within.
Economic Importance. In general the mollusks are of much economic importance. The bivalves especially form an important
source of our food supply. Many of the mollusks also make up an important part of the food supply of bottom-feeding fishes. On the other hand, some mollusks, as Natica, bore into other mollusk shells and eat the animal thus attached. Some boring mollusks, for example the ship worm (Teredo navalis), do much. damage, especially to wharves, as they make their home in piles. Still others bore holes in soft rock and live there.
The shells of mollusks are used to a large extent in manufacture and in the arts, while they form a money basis still in parts of the world. Sepia comes from a cuttlefish.
PEARLS AND PEARL FORMATION. - Pearls are prized the world over. It is a well-known fact that even in this country pearls of some value are sometimes found within the shells of such common bivalves as the fresh-water mussel or oyster. Most of the finest, however, come from the waters around Ceylon. If a pearl is cut open and examined carefully, it is found to be a deposit of the mother-of-pearl layer of the shell around some central structure. It has been believed that any foreign substance, as a grain of sand, might irritate the mantle at a given point, thus stimulating it to secrete around the substance. It now seems likely that perfect pearls are due to the growth within the mantle of the clam or oyster of certain parasites, stages in the development of a flukeworm. The irritation thus set up in the tissue causes mother-of-pearl to be deposited around the source of irritation, with the subsequent formation of a pearl.
CLASSIFICATION OF MOLLUSKS
CLASS I. Pelecypoda (Lamellibranchiata). Soft-bodied unsegmented animals showing bilateral symmetry. Bivalve shell, platelike gills. Examples, clam (Mya arenaria), scallop (pecten), oyster (Ostrea), and fresh-water mussel (Unio).
CLASS II. Gastropoda. Soft bodies asymmetrical; univalve shell or shell absent. Some forms breathe by gills, others by lunglike sacs. Examples, pond snail, land snail (Helix), and slug.
CLASS III. Cephalopoda. Bilaterally symmetrical mollusks with mouth surrounded by tentacles. Shell may be external (nautilus), internal (squid), or altogether lacking (octopus). Examples, squid, octopus.
FOR THE PUPIL
Davison, Practical Zoology, pages 142-150. American Book Company. Herrick, Text-book in General Zoology, Chap. XI. American Book Company. Heilprin, The Animal Life of our Seashore. J. B. Lippincott Company. Jordan, Kellogg, and Heath, Animal Studies. D. Appleton and Company. Morgan, Animal Sketches, Chap. XXI. Longmans, Green, and Company.
FOR THE TEACHER
Bulletin, U.S. Fish Commission, 1889.
Cooke, The Mollusca, Cambridge Natural History. The Macmillan Company.
Vol. XIX, page 193.
Parker, Elementary Biology. The Macmillan Company.
Parker and Haswell, Text-book of Zoology. The Macmillan Company.
THE ordinary stickleback is a hardy fish easily kept in aquaria in the schoolroom. It (or any other small fish, as the brook minnow or goldfish) may be used in the following exercise.1
The Body. The body of the fish runs insensibly into the head, the neck being absent. Notice the long, narrow body. How is it adapted for the method of life? Certain cells in the skin secrete mucus or slime. How might the slimy surface of the body be useful to the fish? If the fish is in an aquarium with surroundings like that of its natural habitat, decide whether the color of the fish is protective.
The Appendages and their Uses. - The appendages of the fish consist of paired and unpaired fins. The paired fins are four in number, and are believed to be homologous with the paired limbs of a man. Compare the fish with the diagram in the book and locate the paired pectoral and pelvic fins. (These are so called because they are attached to the bones forming the pectoral and pelvic girdles. See page 275.) Find, by comparison with the diagram, the dorsal, anal, and caudal fins. How many unpaired fins?
The fins of a fish: A, dorsal; B, caudal; C, anal; D, pelvic; E, pectoral.
The stickleback, as the name indicates, has the front dorsal fin so modified as to form a number of sharp spines. (There are five in the common brook stickleback of New York state.) A careful study of a fish in the school aquarium will help to an understanding of the uses of the different fins. Decide what part in the locomotion of the fish is taken by the caudal fin. Do the other fins function in forward movement? Which fins are used in turning? In backing? Are any used in balancing? Do any parts of the body play a part in locomotion?