IGCSE NOTES : Biology - The animal kingdom

Arthropods

The arthropods include the crustacea, insects, centipedes and spiders. The name arthropod means ‘jointed limbs’, and this is a feature common to them all. They also have a hard, firm external skeleton, called a cuticle, which encloses their bodies. Their bodies are segmented and, between the segments, there are flexible joints which permit movement. In most arthropods, the segments are grouped together to form distinct regions, the head, thorax and abdomen.

Crustacea

Marine crustacea are crabs, prawns, lobsters, shrimps and barnacles. Freshwater crustacea are water fleas,
Cyclops, the freshwater shrimp (Gammarus) and the water louse (Asellus). Woodlice are land-dwelling crustacea.

Like all arthropods, crustacea have an exoskeleton and jointed legs. They also have two pairs of antennae which are sensitive to touch and to chemicals, and they have compound eyes. Compound eyes are made up of tens or hundreds of separate lenses with light-sensitive cells beneath. They are able to form a crude image and are very sensitive to movement. Typically, crustacea have a pair of jointed limbs on each segment of the body, but those on the head segments are modified to form antennae or specialised mouth parts for feeding.

Insects

The insects form a very large class of arthropods. Bees, butterflies, mosquitoes, houseflies, earwigs, greenfly and beetles are just a few of the subgroups in this class. Insects have segmented bodies with a firm
exoskeleton, three pairs of jointed legs, compound eyes and, typically, two pairs of wings. The segments are grouped into distinct head, thorax and abdomen regions.

one pair of antennae and only three pairs of legs. There are no limbs on the abdominal segments. The insects have very successfully colonised the land. One reason for their success is the relative impermeability of their cuticles, which prevents desiccation even in very hot, dry climates.

Arachnids

These are the spiders, scorpions, mites and ticks. Their bodies are divided into two regions, the cephalothorax and the abdomen. They have four pairs of limbs on the cephalothorax, two pedipalps and two chelicerae. The pedipalps are used in reproduction; the chelicerae are used to pierce their prey and paralyse it with a poison secreted by a gland at the base. There are usually several pairs of simple eyes.

These are millipedes and centipedes. They have a head and a segmented body which is not obviously divided into thorax and abdomen. There is a pair of legs on each body segment but in the millipede the abdominal segments are fused in pairs and it looks as if it has two pairs of legs per segment.

As the myriapod grows, additional segments are formed. The myriapods have one pair of antennae and simple eyes. Centipedes are carnivorous; millipedes feed on vegetable matter.

Vertebrates

Vertebrates are animals which have a vertebral column. The vertebral column is sometimes called the spinal column or just the spine and consists of a chain of cylindrical bones (vertebrae) joined end to end. Each vertebra carries an arch of bone on its dorsal (upper) surface. This arch protects the spinal cord), which runs most of the length of the vertebral column. The front end of the spinal cord is expanded to form a brain which is enclosed and protected by the skull. The skull carries a pair of jaws which, in most vertebrates, have rows of teeth. The fi ve classes of vertebrates are fi sh, amphibia, reptiles, birds and mammals.

Body temperature

Fish, amphibia and reptiles are often referred to as ‘cold-blooded’. This is a misleading term. A fi sh in a tropical lagoon or a lizard basking in the sun will have warm blood. The point is that these animals have a variable body temperature which, to some extent, depends on the temperature of their surroundings. Reptiles, for example, may control their temperature by moving into sunlight or retreating into shade but there is no internal regulatory mechanism. So-called ‘warm-blooded’ animals, for the most part, have a body temperature higher than that of their surroundings. The main difference, however, is that these temperatures are kept more or less constant despite any variation in external temperature. There are internal regulatory mechanisms which keep the body temperature within narrow limits. It is better to use the terms poikilothermic (variable temperature) and homoiothermic (constant temperature). However, to simplify the terms, ‘cold blooded’ and ‘warm blooded’ will be referred to in this section. The advantage of homoiothermy is that an animal’s activity is not dependent on the surrounding temperature. A lizard may become sluggish if the surrounding temperature falls. This could be a disadvantage if the lizard is being pursued by a homoiothermic predator whose speed and reactions are not affected by low temperatures.

Fish

Fish are poikilothermic (cold blooded) vertebrates. Many of them have a smooth, streamlined shape which offers minimal resistance to the water through which they move. Their bodies are covered with overlapping scales and they have fins which play a part in movement. Fish breathe by means of fi lamentous gills which are protected by a bony plate, the operculum. Fish reproduce sexually but fertilisation usually takes place externally; the female lays eggs and the male sheds sperms on them after they have been laid.

Amphibia

Amphibia are poikilothermic (cold blooded) vertebrates with four limbs and no scales. The class includes frogs, toads and newts. The name, amphibian, means ‘double life’ and refers to the fact that the organism spends part of its life in water and part on the land. In fact, most frogs, toads and newts spend much of their time on the land, in moist situations, and return to ponds or other water only to lay eggs. 

The toad’s skin is drier than that of the frog and it has glands which can exude an unpleasant-tasting chemical which discourages predators. Amphibia have four limbs. In frogs and toads, the hind feet have a web of skin between the toes. This offers a large surface area to thrust against the water when the animal is swimming. Newts swim by a wriggling, fi sh-like movement of their bodies and make less use of their limbs for swimming. Amphibia have moist skins with a good supply of capillaries which can exchange oxygen and carbon dioxide with the air or water. They also have lungs which can be infl ated by a kind of swallowing action. They do not have a diaphragm or ribs. Frogs and toads migrate to ponds where the males and females pair up. The male climbs on the female’s back and grips fi rmly with his front legs. When the female lays eggs, the male simultaneously releases sperms over them. Fertilisation, therefore, is external even though the frogs are in close contact for the event.

Reptiles

Reptiles are land-living vertebrates. Their skins are dry and the outer layer of epidermis forms a pattern of scales. This dry, scaly skin resists water loss. Also the eggs of most species have a tough, parchmentlike shell. Reptiles, therefore, are not restricted to damp habitats, nor do they need water in which to breed. Reptiles are poikilothermic (cold blooded) but they can regulate their temperature to some extent. They do this by basking in the sun until their bodies warm up. When reptiles warm up, they can move about rapidly in pursuit of insects and other prey.The reptiles include lizards, snakes, turtles,tortoises and crocodiles.

Apart from the snakes, reptiles have four limbs, each with fi ve toes. Some species of snake still retain the
vestiges of limbs and girdles. Male and female reptiles mate, and sperms are passed into the female’s body. The eggs are, therefore, fertilised internally before being laid. In some species, the female retains the eggs in the body until they are ready to hatch.

Birds

Birds are homoiothermic (warm blooded) vertebrates. The vertebral column in the neck is fl exible but the rest of the vertebrae are fused to form a rigid structure. This is probably an adaptation to fl ight, as the powerful wing muscles need a rigid frame to work against. The epidermis over most of the body produces a
covering of feathers but, on the legs and toes, the epidermis forms scales. The feathers are of several kinds. The fl uffy down feathers form an insulating layer close to the skin; the contour feathers cover the body and give the bird its shape and colouration; the large quill feathers on the wing are essential for flight. Birds have four limbs, but the forelimbs are modifi ed to form wings. The feet have four toes with claws which help the bird to perch, scratch for seeds or capture prey, according to the species. The upper and lower jaws are extended to form a beak which is used for feeding in various ways. In birds, fertilisation is internal and the female lays hard-shelled eggs in a nest where she incubates them.

Mammals

Mammals are homoiothermic (warm blooded) vertebrates with four limbs. They differ from birds in having hair rather than feathers. Unlike the other vertebrates they have a diaphragm which plays a part in breathing.They also have mammary glands and suckle their young on milk. Humans are mammals. All mammals give birth to fully formed young instead of laying eggs. The eggs are fertilised internally and undergo a period of development in the uterus.

The young may be blind and helpless at fi rst, e.g. cats, or they may be able to stand up and move about soon after birth, e.g. sheep and cows. In either case, the youngster’s fi rst food is the milk which it sucks from the mother’s teats. The milk is made in the mammary glands and contains all the nutrients that the offspring need for the fi rst few weeks or months, depending on the species. As the youngsters get older, they start to feed on the same food as the parents. In the case of carnivores, the parents bring the food to the young until they are able to fend for themselves.