II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

Most of the chordates in the sea, and all those on the land, are vertebrates. The remaining species — the invertebrate chordate — belong to two subphyla that are extremely dissimilar in outward appearance. The three diagnostic characteristics of the phylum Chordata are: (1) a dorsal, hollow nerve cord; (2) clefts in the wall of the throat region, usually referred to as gill slits, which circulate water during feeding and respiration; and (3) a notochord, a unique, stiffening rod located along the back. All chordates are deuterostomes.

A vertebrate is an animal with a series of bones — vertebrae — that surround the notochord and the nerve cord. In the groups that appeared later in time, the notochord is present only during embryonic stages. Among vertebrates the fishes are the principal large-bodied herbivores, carnivores, and scavengers of the sea and fresh water. Amphibians, reptiles, birds, and mammals play the same role on land. Although low in numbers of individual organisms, vertebrates rival the major invertebrate phyla in diversity and ecological significance.

Vertebrates have a complicated closed circulatory system in which the blood, containing hemoglobin-filled red blood cells, is pumped by the chambered heart through dense system of capillaries. Another trait associated with increased size and activity is the highly developed nervous system. The brain reaches its highest degree of complexity and organization in mammals. A third development in vertebrate evolution is a complex sensory apparatus, including large eyes capable of sharp image perception, ears that serve in some forms as organs of equilibrium and in other forms for both equilibrium and hearing, and, in certain aquatic forms, a lateral-line system that provides a sense capable of detecting slight changes in water pressure and currents.

Fishes. Scientists divide fish into three groups based on their anatomy: jawless fish, cartilaginous fish, and bony fish. Jawless fish are the only living vertebrates that have never evolved jaws. There are about 50 species — a tiny fraction of the world's total fish — and they are instantly recognizable by their sucker-like mouths. Eels, lampreys, and hagfish are examples of jawless fish. Cartilaginous fish do have jaws and use them to deadly effect. Numbering about 1,000 species, they include sharks, skates, and rays, as well as chimaeras, also known as ratfish. Bony fish are some of the most successful vertebrates alive today. These animals can be found in a vast variety of habitats, from coral reefs and the deep-sea bed to lakes hidden away in caves. As their name indicates, bony fish have a skeleton made of bone, and most also have an air-filled sac called a swim bladder that keeps them buoyant. At least 24,000 species of bony fish have been identified, and many more probably await discovery. Common bony fish include salmon, sturgeon, and cod.

The amphibians arose from the crossopterygian-like ancestors in Devonian times. This epochal event did not require a new design of the respiratory system, for the ability to breathe air was already present in the ancestral bony fishes. Instead, the crucial step was the evolution of the stubby fins of the crossopterygians into the walking legs of the amphibians — legs whose basic design has remained unchanged throughout the evolution of terrestrial vertebrates.

Three major orders of amphibians are alive today – the worm-like, tropical, burrowing caecilians; frogs and toads (anurans); and salamanders. Most modern amphibians spend part or all of their adult lives on land, typically in a moist habitat, but return to freshwater to lay their eggs. Amphibian eggs are surrounded by delicate membranes. They contain only limited supplies of yolk, so the larvae must feed soon after hatching. Usually the egg gives rise to an aquatic larva, such as the tadpole of the anurans, that pursues a life in water for some period of time before metamorphosing into a terrestrial adult form. Some salamanders, however, lay their eggs in moist sites on land and never enter the water. Others are aquatic throughout their lives.

Reptiles have a number of prominent adaptations that allow them to exploit terrestrial environment more effectively than do amphibians. Reptiles that live mostly in water (like most turtles) entered this habitat later, as a secondary adaptation. The first reptiles arose from early amphibians some 300 million years ago. The major order of modern reptiles are the turtles, the crocodiles, the squamates (snakes and lizards), and the tuatara (represented by a single species on a few islands of New Zealand).

The liberation of the reptile life cycle from the water depended upon the structure of the reptilian egg, which is essentially identical to the bird egg. It has a leathery or brittle calcium-impregnated shell that resists evaporation of the fluids inside and a number of other features due to which such an egg need not be laid in the water. Some other adaptation of reptiles include the horny keratin scales that cover their skin reducing water loss from the body surface, and the generally larger surface area of their lungs in comparison with the amphibians. The ventricle of the reptilian heart is divided into chambers that separate the freshly oxygenated blood from unoxygenated blood and permit oxygenated blood to be pumped to needy tissue more efficiently. Respiration involves a bellows-like movement of the ribs. The reptilian brain, with small cerebral hemispheres, is more complex than that of amphibians.

The birds. Zoologists sometimes lightly refer to birds as “feathered reptiles” or “hot lizards”, and there is an important truth embodied in these phrases. Virtually all the important differences between the birds and their reptilian ancestors have arisen as adaptations to flight. The single most characteristic feature is their feathers, which are highly modified versions of reptilian scales. The major modifications in the body skeleton of the birds include light and strong hollow bones with internal struts and a conspicuous change in the shape of the sternum (breast bone), which was transformed into a large vertical keel for the attachment of the breast muscles. A flying bird has a very high metabolic rate that requires a highly efficient circulation, which has been achieved by the complete division of the ventricle into two chambers. One chamber pumps “used” blood to the lungs; the other receives freshly oxygenated blood from the lungs and pumps it to the rest of the body. The lungs have a flow-through pattern that allows a more complete exchange of respiratory gases than does the pattern of mammalian lungs. The avian brain is much larger in proportion to body size than that of reptiles. The difference is not in the cerebral cortex, the principal seat of intelligence in mammals, but rather in the cerebellum, the centre of sight and muscular coordination.

The features considered to be diagnostic of mammals include well-developed sensory capabilities and intelligence. Only mammals suckle their young with nutritive fluid. Other characteristics contributing to the success of mammals in many ecological situations include: (1) teeth variously specialized for cutting, chewing, and grinding: (2) the diaphragm, a muscle wall that completely separates the chest cavity from the abdominal organs and increases the depth and efficiency of breathing; (3) hair, providing mechanical protection and an aid in heat conservation; and (4) a greatly enlarged brain, chiefly in the cerebral hemispheres, allowing mammals both more complex instincts and more elaborate learning.

Читайте також:

<== попередня сторінка	\|	наступна сторінка ==>
I. Match each word on the left to its correct definition on the right	\|	Vocabulary notes

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.006 сек.