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 the squid. 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. REFERENCE BOOKS FOR THE PUPIL American Book Company. Davison, Practical Zoology, pages 142-150. FOR THE TEACHER Bulletin, U.S. Fish Commission, 1889. Brooks, The Oyster. Johns Hopkins Press. Cooke, The Mollusca, Cambridge Natural History. The Macmillan Company. Kellogg, The Life History of the Common Clam. Bulletin, U.S. Fish Commission, Vol. XIX, page 193. Parker, Elementary Biology. The Macmillan Company. Parker and Haswell, Text-book of Zoology. The Macmillan Company. XXII. FISHES 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 do you find? 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? (Notice the muscular body ending in the tail.) The Senses. -Notice the position of the eyes from the front and in a side view of the fish. Is the position of advantage and how? The eye is 1 See Hunter and Valentine, Manual, page 167. globular in shape. Such an eye has been found to be very near sighted. Thus it is unlikely that a fish is able to perceive objects at any great distance from it. The eye is unprotected by eyelids, but the position on the sides of the body affords some protection. There is much opportunity for a careful boy or girl to make simple experiments to determine how much and how far the fish can see. Test to get reactions to light of various intensity, to moving objects, to shadows, etc. Feed the fish; does it appear to see the food or to go to the food by a sense of smell? The nostrils of a fish can be proved to end in little pits, one under each nostril hole. Thus they differ from our own, which are connected with the mouth cavity. In the catfish, for example, the barbels or horns receive sensations of smell and taste. The sense of perceiving odor is not as we understand the sense of smell, for a fish perceives only substances that are dissolved in the water in which it lives. The senses of taste and touch appear to be less developed than the other senses. A fish rejects bits of food which it does not like; we all know that different kinds of bait appeal to different kinds of fish. Breathing. Notice that a fish, when swimming quietly or when at rest, seems to be biting when no food is present. A reason for this act is to be seen when we introduce a little finely powdered carmine into the water near the head of the fish. It will be found that a current of water enters the mouth at each of these movements and passes out through two slits found on each side of the head of the fish. Investigation shows us that h under the broad, flat plate or operculum forming each side of the head lie several long, feathery, red structures, the gills. Gills. If we examine the b gills of any large fish, we find that a single gill is held in place by a bony arch, made of several pieces of bone which are hinged in such a way as to give great flexibility to the gill arch, as the support is called. Covering the bony framework, and extending into the throat, are a series of delicate filaments of flesh, covered with a very delicate membrane or skin. Into each of these filaments pass two blood vessels, one downward and one upward. Blood reaches the gills and is carried away from these organs by means of two large vessels which pass along the bony arch previously mentioned. Blood passes into the gill filament, and there comes in contact with the free oxygen of the water bathing the gills. An exchange of gases through the walls of the gill filaments results in the loss of carbon dioxide and a gain of oxygen by the blood. Possibly other waste products find their way into the water bathing the gill filaments, the wastes being carried off by the current of water passing over the gills. Gills and heart of the perch, exposed by removal of gill cover on left side; a, first of the four bony arches which carry the gills; b, gills; b', lower edges of gills on the right side; h, heart. Teeth. Notice the arrangement and number of the teeth in the stickleback. Sticklebacks are carnivorous, preying upon the eggs and young of other small fish inhabiting the brooks where they live. They also feed upon decaying and live vegetable matter, especially algæ. The teeth of the stickleback are small and evidently useful for seizing and holding prey. This fish, like many other species, is a hunter and uses the teeth as weapons of offense as well as for defense. How are the teeth of the stickleback adapted to their functions? The tongue in most fishes is wanting or very slightly developed. GILL RAKERS.-If we open wide the mouth of any large fish and look inward, we find that the mouth cavity leads to a funnel-like opening, the gullet. On each side of the gullet we can see the gill arches, guarded on the inner side by a series of sharp pointed structures, the gill rakers. In some fishes in which the teeth are not well developed, there seems to be a greater development of the gill rakers, which in this case are used to strain out small organisms from the water which passes over the gills. fishes, as the shad and menhaden, make such use of the gill rakers. Many DIGESTIVE SYSTEM.-The gullet leads directly into a baglike stomach. There are no salivary glands in the fishes. There is, however, a large liver, which appears to be used as a digestive gland. This organ, because of the oil it contains, is of considerable economic importance. Most fishes have a series of pockets, called pyloric cæca, which are outgrowths from the intestine and probably serve to give more absorbing surface to that part of the digestive tract. The intestine ends at the vent, which is usually located on the ventral side of the fish, immediately in front of the anal fin. Anatomy of the carp; br, branchiæ, or gill openings; c, heart; f. liver; vn, swimming bladder; ci, intestine. SWIM BLADDER. An organ of unusual significance, called the swim bladder, is connected with the digestive tract in front of the stomach. In young fishes of many species this connection is a tube, which in some forms HUNTER'S BIOL.- -18 |