These were of no great value at first but later, as the climate grew drier, these animals survived better than those without the adaptations. In other words they were adapted to the drier conditions.

Adaptive Radiation - The gradual spread of a type of animal into a number of different habitats with consequent changes in structure and appearance. The mammals, and the reptiles before them, radiated on land and into the air and water, producing from a basic stock such varied animals as gazelles, bats, and whales. A more recent example is that of Darwin' s finches a group of birds inhabiting the Galapagos Islands. There is a basic similarity between the various species but the beaks vary from the heavy seed crushing type to the narrow insect eating type. It is believed that a single ancestral type of bird arrived in the islands not too long ago and has since radiated into a number of distinct feeding types. The radiation was possible because variation in the original birds meant that some were adapted better to a diet of seeds and others to a diet of insects.

Adrenal Gland - In man, the adrenal glands are loosely attached to the upper side of the kidneys. The combined weight of these tiny yellow glands is less than one ounce but they play an important part in the regulation of the body's activity. Each adrenal is actually composed of two distinct parts a central medulla and an outer cortex. The origins and functions of the two parts are very different adrenal medulla resembles nervous tissue in its action: the nerves supplying it actually make contact with the medulla cells and control their secretion of adrenaline. This hormone is released in response to nervous signals when one experiences fright or mounting tension. Its action prepares the body for any sudden strain by increasing the blood supply to the brain and muscles. The heart beat is accelerated and the skin pales. These symptoms can be experienced when someone steps out in front of your car or when you are about to sing your first solo in public. The bristling of a cat's fur when confronted by a dog is also due to the action of adrenaline.The adrenal cortex also aids the body to prepare for or adapt to undue stress but the action in this case is not immediate. The secretions of the cortex are fatty substances called steroids and they are concerned with the production of energy in the cells of the body. Steroid production is triggered off by chemical messengers (hormone) from the pituitary gland.

The main hormone concerned is adrenocorticotropic hormone (ACTH). In a helthy person there is a delicate balance between the supply and demand of steroids from the cortex. The cells need more or less steroids according to the conditions. Prolonged cold, low air pressure, pregnancy, illness and many other features, all alter the demand for steroids.

If there is more than the necessary amount of steroids in the blood, they will not be used. Steroids returning in the blood to the pituitary will prevent the production of ACTH and thus the production of more steroids will be halted. Similarly if not enough steriods are being produced, none, or only a low concentration, will reach the pitutiary and ACTH will be released. This in turn causes increased production of steroids by the adrenal cortex. ACTH is also released by the action of adrenaline on the piturtary. Adrenaline therefore not only prepares the body for immediate action but also helps in the preparation for longer periods of stress.

Adrenaline - The secretion of the adrenal medulla. (See Adrenal Gland).

Adrenocorticotrople Hormone (ACTH) - Hormone produced by the pituitary gland controlling secretion of the adrenal cortex. (See Adrenal Gland).

Aerobic - Requiring free oxygen for respiration.

Aestivation - Animals in many of the drier parts of the world hide away in the summer or driest season and sleep, thereby avoiding the danger of drying up. This is known as aestivation. (See Hibernation).Afferent. Leading towards. Used especially of arteries leading towards the gills of fishes.

Agnatha - Group of primitive vertebrates without jaws. Most of the group are extinct but the class Cyclostomata contains a few living forms the lampreys and hagfishes. The extinct members all fish like probably fed by sucking up mud and filtering out edible particles.

Air Bladder - (=Swim Bladder).

Albinism - Lack of pigment in the skin, resulting in pure white individuals.Albinos have pink eyes because there is no pigment in the iris to mask the colour of the blood capillaries.

Alimentary Canal - The food canal or gut of an animal running from the mouth to the anus. The detailed structure varies enormously among the many groups of animals but is fairly constant within each group, with certain modifications according to diet. The mammalian alimentary canal follows a fairly standard pattern. The mouth opens into the buccal cavity which houses the tongue and teeth and leads back to the from the pharynx into which open the nasal cavity and the windpipe.

From the pharynx a long muscular tube called the oesophagus runs down through the thorax and opens into the stomach. This is a muscular bar in which the food stays for a time and is thoroughly mixed up. Some chemical breakdown occurs in the stomach. The stomach of cows and other ruminants is divided into several compartments where cellulose is broken down by bacteria. Food is usually swallowed straight into the first compartment (the rumen) and later reguritated for mastication chewing the cud.

To return to the typical mammal, the stomach leads by way of a muscular valve into the small intestine a long coiled tube where most of the digestion and absorption of food occurs. The inside wall of the small intestine is thrown into numerous folds and finger like projections called villi. These increase the surface area or efficient absorption. (Sharks have only a short intestine but its effective length is increased by a spiral valve (See Elasmobranchii). The earthworm increases the surface area of its intestine by means of an infolding called the typhlosole.

(See Oligochaeta). The small intestine passes into the large intestine which is concerned largely with the reabsorption of water from the undigested food. At the junction between the small and large intestines there is a blind sac called the caecum. In man and many other mammals the caecum is very small and at its blind end has an even smaller appendix, but in horses, rabbits, and some other animals the caecum is very large and is the site of cellulose digestion by bacteria. The large intestine leads to the rectum a short tube that passes undigested mattery (faeces) to the anus.

The alimentary canal of a bird is somewhat different from that of a mammal. The oesophagus leads to the crop, especially well developed in grain eating birds, where food is stored and partially broken up. The stomach is in two parts the proventriculus and the gizzard. The former produces enzymes and mixes them with the food which then passes to the gizzard, a very muscular region where the food is ground up. Many birds swallow stones which get trapped in the gizzard and help in the grinding work. Flesh eating birds do not have such a muscular gizzard

Allatois - A structure present in the embroyos of amniote vertebrates. It is an outgrowth of the gut and contains many blood vessels. In the eggs of reptiles and birds the allantois grows near to the shell and oxygen can diffuse into its blood vessels and thus reach the embryo. The mammalian aliantois mingles with the maternal tissues of the placenta and receives oxygen and food from the maternal blood vessels.

Allele - (See Gene).

All or Nothing Law - A feature of muscle and nerve cells. When such cells are stimulated they either react completely or not at all, depending on the intensity of the stimulus. There is no partial reaction.

Allen's Law - The recognised fact that animals living in cold climates have smaller ears, snouts, etc., than related animals in warmer lands. The reduction of these parts reduces heat loss and is therefore an advantage.

Alternation of Generations - The existence of two distinct forms in the lifecyele of an organism, both of which forms can reproduce. One form reproduces sexually and gives rise to the other form which in turn reproduces asexually to give the sexual form again. This phenomenon occurs in most plants but is less common in animals, being best shown by the coelenterates. Obelia is a colonial animal with a lot of Hydra like polyps all joined together. Special reproductive polyps produce tiny jelly fish (medusae) by budding and the medusae swim off into the water. Each medusa produces either male or female cells which are released and join together to grow into a new polyp colony. The polyps are the asexual generation and the medusae the sexual generation.