Epigamic Character - A feature other than the sex organs and their associated structures, that is concerned with sexual reproduction. For example, the splendid tail of a peacock that is believed to attract a mate.

Epiglottis - Flap of cartilaginous tissue that closes the windpipe opening (glottis) when food is swallowed.

Epiphysis - Head of a long bone.

Epipubie Bone - (See Marsupialia).

Epithelium - A sheet of cells held together by a small amount of cementing substances. The skin the lining of the gut and other organs, such as the lungs and blood vessels are examples. Below most epithelia there is a thin sheet of connective tissue, the basement membrane. The free surface (the surface which is not attached to other tissue) is often cilated. When the epithelium has several layers of cell& it is said to be stratified.

The cells of epithelia may serve very different purposes. Those lining the salivary glands and the glands in the intestine for example, produce digestive enzymes. The ones forming the outer covering of the skin are mainly protective, while the cells of the lung lining produce the wet mucus in which the oxygen dissolves before passing to the blood. The cells may vary in shape and size. Some are thin and flat like crazy paving stones.

They form pavement or squamus epithelium, which is found for example in the lining of parts of the kidney tubes. When the of squamous epithelium have wavy outlines (e.g. cells lining the blood vessels) they are said to be tesselated. Other cells are approximately as wide as they are tall. These form cuboidal or cubical epithelium which is found in many glands (e.g. liver). In columnar epithelium the cells are tall and column shaped. Such epithelium lines most of the gut.

If columnar cells bear cilia the epithelium is then called cliated columnar epithelium. Ciliated cells occur in the lining of the windpipe. The cilia beat to help remove dirt partiles.

The outer cells of the skin and the lining of the cheek form stratified squamous epithelium.


Erythrocyte - Red blood oorpuscle.

Erythrism - Excess of red pigment in the body.

Eurypterida - An extinct order of aquatic arachnids some of which teached a length of six feet. They were like scorpions in some ways and the larger ones were probably fierce predators.

Eustachian Tube - Tube running from the middle ear to throat of most tetrapods. Enables pressures on each side of me ear drum to remain equal.

Eutherian - A placental mammal.
Evolution - The process whereby living things are believed to have arisen from less advanced forms by gradual change. A great deal of evidence supports the idea of evolution, especially the evidernce provided by the fossil record. Darwin and Wallace put forward a theory of how these gradual changes could be passed on to succeeding generations.

Excretion - Living organisms burn up fuel to produce energy needed for movement, growth, and the replacement of worn out tissues. This burning process goes on in the cells and produces waste material just as a fire in the hearth produces ash and smoke. The waste; materials must be removed from the body otherwise they would poison it. Carbon dioxide and the main waste products. Carbon dioxide diffuses out of the body or may be expelled through the breathing organs. While, this is a form of excretion the term is normally restricted to the breakdown and removal of nitrogenous waste.

The main nitrogen containing waste substance that must be removed from an animal's body is ammonia. This is mainly formed when organic food materials containing nitrogen, that are surplus to the body's requirements, are used as fuel. Ammonia is a very poisonous substance and must be removed rapidly or converted into a less harmful substance (e.g. urea) CO(NH₂)₂.

In many simple animals that live in water ammonia seeps (diffiuses) out of the body in solution (ammonia dissolves redily in water). Its removal in this way is rapid and efficient. Our present knowledge is that most protozoa excrete mainly ammonia compounds. Urea and, to a lesser extent, urates (salts of uric acid) are also e reted by some. In many protozoa there are one or more water tilled spaces or contractile vacuoles. These swell in size, pumping out water from the surrounding protoplasm, and then burst, releasing their contents to the exterior. It is possible that waste materials may be dissolved in the fluid discharged, and that this may be a means of removing them.

More complicated animals have special excretory organs. 'The excretory system of most flatworms consists of a pair of canals on either side of the body opening in places to the exterior. The canals branch many times before ending in the excretory organs, tiny structures called flame cells. Within the cavity of each flame cell is a bundle of hairs which flicker like a flame (hence the name 'flame cell') and as a result water, waste materials and other substances pass into the flame cell cavity. Lower down the tube, useful susbstances are reabsorbed and the 'urine' passes to the exterior, Primarilly, however, the flame cells are concerned with regulating the fluid content of the flatworm.

In ringed worms or annelids (e.g. the earthworm) the excretory organs are called nephridia, of which there is one pair per segment. The nephridia also regulate the content of the body fluids. In the earthworm the intestine is covered with yellow cells that extract nitrogen containing waste matter (guanin) from the bloodstream. When they are full of waste they break up and float in the coelomic fluid, the finest particles being carried to the exterior through the nephridia. The nephridia have a good blood supply and the middle part of the tube extracts urea and ammonia from the blood, passing them out in the urine. Since the strength of the urine below that of the blood unwanted water must also be removed. The funnel of each nephridium is not open all the time. Each has a, ring muscle that opens at intervals to allow fluid to escape. Waste also passes form the body f1uid into the gut, carried in special amoeboid cells.

The main excretory organs of crustaceans are two pairs of glands near the antennae and mouthparts. These are rarely present. together, often one pair serves as the 'kidneys' in the larva arid the other in the adult. For example, in crabs and lobsters the antennal glands are the kideys of the adult, but in most adult crustaceans the pair associated with the mouthparts is present.

The excretory structures consist basically of an enlarged sac (end sac) and a duct leading to the exterior. Water, salts and waste substances (e.g. ammonia salts) pass into the end sac and the upper parts of the tube. Valuable salts are reabsorbed in the middle part of the tube to produce a very weak urine containing much water. The organ thus serves for excretion and the removal of unwanted water that enters through the gills. The latter function of the excretory glands is most important in freshwater animals and since the urine is so dilute, highly poisonous substances such as ammonia have no harmful effects while they are within the excretory system.

The formation of a hard outer shell in crustaceans and insects is another means of excretion. The shell contains a large proportion of chitin, a compound with a similar structure to cellulose, but into which ammonia is incorporated. When the shell is cast off during a moult the ammonia is removed.

Most insects also have a well developed system of excretory tubes Malpighian tubules which lead into the junction between the mid gut and hind gut. Uric acid crystals have been found within the Malpighian tubules, showing that they are responsible for removing waste materials from the blood. The tubules also reabsorb water. This may take place in special cells scattered along their length or the cells may form the lower half of each tube. There is also evidence that the urine changes from an alkalineto an acid solution as it passes down the tubulesso that they play a part in regulating the pH (alkalinity and acidity) of the blood, as do the tubules of the human kidney.

Other means of excretion in insects include the deposition of uric acid crystals in such structures as wing scales (e.g. white butter therefore. The water proof cuticle flies).

The majority of insects live on land and, being small, have a largsurface/volume ratio. The need to conserve water that they can obtain by drinking or in their food is critical, is of great importance in reducing loss through evaporation.

The kidneys are the main excretory organs of vertebrates. Each kidney consists of a number of tubules made up of a capsule and tube. The capsule contains a knot of blood capillaries from which water and waste, together with some useful materials, pass into the tube. Various parts of the tube reabsorb water and useful salts. Because different animals live under different conditions some need to be more careful about water loss than others and so details of kidney structure vary.

Freshwater fishes face a similar problem to freshwater invertebrates. Their body fiuids are more concentrated than the water in which they live. Thus they tend to imbibe water and lose salt. Various special modifications restrict these tendencies. Bony fishes have a covering of waterproof scales, and in many (e.g. eels) the skin produces large quantities of slime. The entry of water into the 'body is therefore restricted and the kidneys have extra glomeruli for removing large amounts of water. Sea fishes have fewer glomeruli and conserve water. They also swallow sea water and get rid of the extra salt through special salt secreting cells in the gills. Nitrogenous waste may also be removed in the gills.

Frogs have a moist skin and, when they return to water to breed, large quantities of water must pass in through it. Their kidneys have many glomeruli (the tubules are short) and measurements on the common frog show that they may produce about one third of an ounce of urine per day a third of their weight. Adult frogs excrete urea and so lose less water than their eggs and tadpoles which excreteammonia.

Reptiles and birds live in relatively dry surroundings and most of them excrete uric acid. The kidney capsules are small and only small amounts of water are filtered off. Mammals excrete urea but the kidney tubules have a reabsorbing loop and porduce a concentrated urine.