THE animal kingdom can be conveniently separated into two sections, the Protozoa or unicellular animals, and the Metazoa or multicellular animals. To the latter section belong the coelenterates, flatworms such as the tapeworm and liver fluke, earthworms, crabs and other crustaceans, insects, spiders, slugs and snails. All these have no internal bony skeleton, and where a skeleton is present it is generally external as in the crab. Many Metazoan animals, as, for instance, the earthworm, have practically no hard parts at all. The higher Metazoan animals are placed in the group Vertebrata, and are characterized by the possession of an internal jointed bony skeleton. Part of this skeleton is a central axis termed the backbone or vertebral column, which consists of many small bones called vertebra?. The Metazoan animals which do not possess a backbone are known as the Invertebrata. Other features which mark off the vertebrates from the invertebrates are the possession of ’gill ’slits, either in the embryonic or larval stage only or throughout life, and the presence of a single, hollow, dorsal nerve cord. The vertebrates are divided into five classes :—
Frogs, toads, newts, and salamanders.
Lizards, snakes, turtles, tortoises and crocodiles.
Kangaroos, whales, horses, cats, dogs, seals, bats, rabbits, monkeys and man.
Structural Features of the Vertebrates
The body of a vertebrate is bilaterally symmetrical, I.e. if it is cut into two in the median vertical plane, each half is similar. The body is divided into three regions : the head, the trunk and the tail. In many vertebrates there is a narrow region between the head and the trunk called the neck. The head contains the brain, eyes, auditory and olfactory organs ; the trunk contains the heart, digestive, respiratory, excretory and reproductive organs. The trunk of a fish bears fins, but in the case of the other classes of vertebrates limbs are attached to the trunk. In fishes the tail is thick and muscular and forms the chief locomotor organ, but in the other vertebrates it is usually smaller and less important.
The muscular wall of the trunk encloses a cavity known as the coelom, which contains a fluid and is lined by a membrane known as the peritoneum. The peritoneum projects from the dorsal wall of the coelom in a double fold called the mesentery, which encloses the stomach, intestines, liver and pancreas. These organs are therefore hanging in the coelom suspended by the mesentery. They are protected from injury by the wall of the trunk and are free to slide over each other, being lubricated by the coelomic fluid. The kidneys are not suspended by mesenteries, but lie against the dorsal wall of the coelom and are covered by the peritoneum on their ventral surfaces.
The dorsal part of the ccclomic wall is thicker than the rest, and the vertebral column lies embedded in it. The heart is surrounded by a special part of the peritoneum, which is two-layered and is called the pericardium. The inner layer of this membrane covers the heart closely, but the outer layer fits loosely round it so as to enclose a space, the pericardial cavity. In the lung-breathing vertebrates each lung is surrounded by a similar double-layered part of the peritoneum termed the pleura. In mammals the coelom is divided by a partition, the diaphragm, into two parts ; these are an anterior space termed the thoracic cavity, and a posterior space, the abdominal cavity. The thorax contains the lungs and heart, while the abdomen contains the digestive organs, kidneys and reproductive organs. In fishes the coelom is also divided into two parts, which are a small pericardial cavity enclosing the heart and a large abdominal cavity containing the other viscera.
The Characteristics of the Classes of Vertebrates
Fishes have paired and unpaired fins supported by a skeleton of rods termed fin rays. They breathe by gills and have a scaly skin.
The following classes are characterized by the possession of limbs :—
Amphibians have a soft moist skin, and when adult they breathe by lungs. The young are unlike the adults and breathe by gills ; they also have a long tail with two fringe-like fins, but these differ from those of fish in lacking fin rays.
Reptiles have a dry skin covered by horny scales. They breathe by lungs and the young resemble the adults.
The fore limb of a bird is modified to act as a wing, and the skin bears feathers except on the legs, which are covered with scales.
The skin of mammals is covered with hair. Mammals bring forth their young alive andthem with milk provided by special glands termed mammary glands. Two primitive mammals, the Duck-bill Platypus and the Spiny Anteater, lay shelled eggs like those of birds.
Fishes, amphibians and reptiles are cold-blooded animals, I.e. their body temperature varies with the temperature of their environment, being the same as that of the surrounding air or very close to it. Birds and mammals are warm-blooded animals, for their bodies are kept at a constant temperature, which varies only to the extent of a few degrees and is independent of the surroundings.