The nucleus can be clearly seen with the microscope as a darkish spot inside the body. It controls the whole course of life and an amoeba deprived of its nucleus soon dies. The nucleus is especially important during reproduction. This takes place when food is plentiful and amoeba can grow do tis full size. Then the nucleus divides followed by division of the rest of the cell. The two new amoeba increase in size and may themselves divide.

During droughts or severe cold, amoeba withdraws all pseudopodia and secretes a tough coat or cyst. The cyst with its contents is called a spore. Each spore is very light and may be blown by the wind to new surroundings. Inside the cyst, the original animal divides to form numerous smaller individuals (amoebulae). In favourable conditions the cyst breaks down and releases them.

Amoeba has no special sense organs. Changes in the outside world are detected by all parts of the living material. In general, the sensitivity ensures favourable surroundings. For instance, amoeba quickly moves away from very bright light or strongly acidic or alkaline water.

Amoeboid - Moving and changing shape in the manner of Amoeba.

Amphibia - A class of vertebrate animals whose members are capable in general of life both in and out of water. The class contains the frogs and salamanders and a host of extinct forms very unlike any living amphibian. Amphibians are cold blooded, that is their temperature varies with that of the surroundings and the majority have to return to the water to breed.

Amphibians were the first vertebrates to conquer the land, evolving from a type of lung fish in the Devonian period some 300 million years ago. These early amphibians are prouped in the sub class Stegocephalia which contains only extinct members such as Ichthyostega and Eryops. They were stil fish like in many ways with scaly skins, but they possessed the typical characteristic of land living vertebrates the five fingered (pentadactyl) limb. The stegocephalians were very heavily boned, a feature that has been lost in modern amphibians.

Living amphibians fall into three sub classes: Urodela, the tailed newts and salamanders; Anura, the jumping frogs and toads; and the limbless Apoda burrowing creatures resembling earthworms in outward appearance. Adult amphibians have lungs in most cases but many of them also breath through the moist skin. Eggs are normally laid in water where they hatch into larval forms known as tadpoles. But there are many variations in the life history. Some species lay eggs in damp soil and small adults emerge from the eggs. Others give birth to active young resembling the parents in everything except size the whole larval period is spent inside the mother.

Amphineura - A small class of Mollusca.

Anaerobic - Able to survive in the absence of free oxygen. Anaerobic respiration involves the liberation of energy from materials without using up oxygen.

Analogous Organs - Structures in different orgnisms which perform the same function but which develop in different way. As an example, the wings of the humming bird and of the humming bird hawk moth are used to hover in front of flowers, but they are structurally very different.

Anapsida - A sub class of the Reptilia including the tortoises.

Anatomy - The study of the structure, both internal and external, of animals.

Androgen - Any substance that promotes the development of male sexual characteristics, such as the comb of a cockerel. Androgens are formed mainly in the testis.

Annelida - This phylum of animals contains the segmented worms earth worms, bristle worms, leeches; and others whose bodies are normally well divided into segments or rings. There is a muscular body wall covered in most members by a thin cuticle. The nervous system is well developed and there is a blood system and a large coelom. Of the six classes of annelids the most impotant are: Polychaeta mainly marine animals with many bristles (chaetae) arising in each segment; Oligochaeta earthworms and various freshwater forms with few bristles per segment; and Hirudinea- the leeches which have no bristles.

Ant - A member of the superfamily Formicoidea of the insect order Hynenoptera. All ants are social insects living in large colonies and they are remarkable for the large number of different forms (castes) that exist queens, workers, males, soldiers, etc. Ants can be recognised by the narrow pedicel or 'waist' joining thorax and abdomen and by the characteristically bent antennae.

Antagonism - Conflict between one organism and another, especially with reference to production of antibiotic substances by one organism that interfere with the other. Also used to refer to muscles with opposing actions.

Anteaters - (See Edentata).

Antenna - Sensory organ on the head of various arthropods. It is the first appendage of insects and myriapods, the second of crustaceans. There is great variation in shape. Antennae generally carry organs oncerned with the senses of smell and touch but in some crustaceans they are used as oars to propel the animal through the water.

Antennule - The first appendage of the crustacean head, Usually sensory.

Anthropoidea - Sub order of the Primates containing monkeys, apes and man Antibody. A substance (normally a protein) which is produced in the body of an animal in response to certain foreign substances, such as bacteria that enter the body. Substances that can induce antibody formation are called antigens. Each antigen has one or more specific antibodies that are formed and combine with the antigen to render it harmless or more easily dealt with by the infected animal. Vaccination against a particular disease invoves giving a weak dose of the disease, or an extract from the disease-causing organisms. Antibodies are then formed and remain in the blood. If the disease is picked up the antibodies destroy it before it does much harm.

Antigen - (See Antibody).

Anura - The order of Amphibia containing frogs and toads, all of which are modified in some degree for jumping. Toads tend to be more fully terrestrial than frogs but frogs of the genus Rana are perhaps the most successful of the amphibians. They have an almost world wide distribution and live in a great variety of habitats. The common frog of Great Britain is Rana temporaria and shows the typical features of the group. The long hind legs are well adapted for jumping and for swimming. The moist, soft skin is well supplied with blood vessels and it is an important site of oxygen uptake for respiration (Some anurans have actually done away with their lungs and rely entirely on the skin for gaseous exchange). There are many glands in the skin, producing mucus which keeps the skin moist, and various poisonous substances. Poison glands are better developed in toads than in frogs. Internally, the skeleton is very reduced when compared with the early amphibians, especially in the skull.

Adult frogs. spend the cold winter months in a state of inactivity hidden under logs and stones, or buried in mud. They awaken in early spring and return to the water. As a rule, the males return first and, by their croaking, attract the females. Pairing takes place in the water without any courtship display. As the female discharges her eggs the male releases sperm over them and fertilisation follows. The eggs are covered with jelly which quickly swells on contact with the water. It keeps the eggs together and protects them.

After fertilisation each egg cell begins to divide and, within a few days, the black sphere elongates and develops a head and tail. The tadpole hatches in about ten days and attaches itself to a plant by means of a sticky gland. The mouth is not yet fully formed and the young tadpole exists on the yolk material from the egg. About three days after hatching, the mouth opens and the tadpole then begins to feed on algae. By this time three pairs of delicate external gills have developed.

During the next few weeks the tadpole grows rapidly and important changes take place both internally and externally. Internal gills are formed. These are respiratory openings connecting the mouth and the outside. A fold of skin (the operculum) covers them and opens at a single point (the spiracle) on the left hand side. The external gills disappear, and apart from its large head the tadpole is like a fish. However, the legs soon begin to develop. The hind ones appear first because the front limbs are covered by the operculum. The left front limb appears through the spiracle, followed later by the right leg which breaks through the operculum. Lungs have formed by now and the tadpole begins to breathe air at the surface. It feeds on animal material now, insect larve for example and the mouth enlarges.

As the legs grow, the tail shortens and the typical frog shape becomes obvious. The frog has now metamorphosed or changed into the adultform. It leaves the water and lives in damp vegetation feeding on insects, slugs and worms. The whole process from egg to metamorphois takes about three months but the small frog requires another three or four years to become mature and able to breed.