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The best title for this passage is ______.
A．Adoptive Divergence
B．Evolution
C．Unusual Structures
D．Changes in Organs
Fishes
The term fish is applied to a variety of cold-blooded aquatic vertebrates of several evolutionary lines. Fish share certain features with other vertebrates. These features are gill slits (鳃裂) at some point in the life cycle, a notochord (脊索), or skeletal supporting rod, a dorsal hollow nerve cord, and a tail.
The study of fishes, the science of ichthyology (鱼类学), is of broad importance. There are many reasons why fishes are of interest to humans; the most important is their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world's food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean.
Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito borne diseases. Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates.
There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. To many, aquarium fishes provide a personal challenge, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing support multimillion dollar industries throughout the world.
Fishes have been in existence for more than 450,000,000 years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes, for when fishes colonized the land habitat they became tetrapod (四足的) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform. to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.
Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration, some for the purpose of camouflage, others for the enhancement of behavioral signals.
Fishes range in adult length from less than 10 millimetres to more than 20 metres and in weight from about 1.5 grams to many thousands of kilogr
A．Y
B．N
C．NG
Whales
鲸
Whales are aquatic mammals belonging to order CetaceA．A few species live in fresh water, but most species live in the seA．They have a streamlined shape and a powerful tail to drive them forwarD．With its two large horizontal fins or flukes, the tail produces the driving force by beating strongly up and down. Flippers at the front are used for steering and balance. The hind limbs of whales have completely disappeared, apart from a few small bones inside the body. Body hair has also disappeared, giving whales a smoother outline and less resistance to water.Instead of hair, whales are insulated by a thick layer of fat, or blubber, under the skin. The blubber may be as much as 61 cm thick on some parts of the body. Besides protecting the animal against the cold, the blubber is an important food reserve.
Most of the best-known whales large creatures.For example, the blue whale reaches a length of more than 30 m.However, many whales, such as dolphins and porpoises, are small.Some are only 1.5 to 1.8 m long.
Whales live entirely in water.Sometimes, whales are stranded on the shore.Although they are air-breathing animals, they soon die because their great weight keeps them from expanding (opening out) the chest cavity.They can breathe easily when afloat, because the water supports most of their weight.
The bottle-nosed whale has been known to stay under water for about two hours.The sperm whale can dive down to depths of 500 fathoms.Such long and deep dives are unusual.Most dives last between 10 and 30 minutes.Whales have special mechanisms that help them to stay under water.When they breathe, they renew about nine-tenths of the air in their lungs.When human beings breathe in, only about one-fourth of the air is renewed.Whales therefore have a fairly large supply of fresh air to start with.They also have an additional oxygen supply in the muscles, where air is loosely held in combination with a pigment called myohaemoglobin.Another thing that helps whales hold their breath for long periods of time is their low sensitivity to carbon dioxide in the blood.(It is the carbon dioxide building up in human blood that affects the brain and makes the human being take another breath.)
When a whale surfaces to renew its air supply, it needs only to push the top of its snout out of the water.This is because the nostril or blowhole is at the top of the heaD．The expelled air is forced out strongly to form. the spout or blow.
Living whales are divided into two groups—tooth whales and whalebone whales.Tooth whales, which include most species, generally have many conical teeth and eat mainly squids and fishes.The killer whale feeds on seals.One African river dolphin feeds mostly on plants.Other tooth whales include the narwhal and the sperm whale.
Whalebone whales have no teeth.Their mouths contain huge comblike fringes of baleen or whalebone.This horny substance is usually black.All whalebone whales feed by straining small animals from the water.The mouth is filled with water and the water is then forced out through the fringes by the tongue.The animals caught in the baleen are swept into the stomach by the tongue. All whalebone whales are large animals, which are usually found in cold seas.They include the blue whales, the right whales, and the rorquals.
The future of many of the larger kinds of whales is uncertain. Whalers have killed so many blue, bowhead, humpback, and right whales that those species are threatened with extinction. Overhunting has also greatly reduced the number of fin and sei whales.Also, if the human population does not stop increasing, people may have to compete with whales for food in the seA．Some nations have begun fishing for krill.Krill is the chief food of wha
A．good bodily resistance to water
B．only a few small bones inside the body
C．neither hind limbs nor body hair
D．a thick layer of fat under the skin
One example of physically induced changes of aquatic habitats is
A．easier boating.
B．more fishing.
C．herbicide use.
D．lack of aquatic plants.
A.transpiration
B.aquatic
C.resistance
D.infiltration
All the aquatic mammals sleep a lot.
A．Right
B．Wrong
C．Not mentioned
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