Rectum- Last part of alimentary canal.

Reffex Action- A simple form of behaviour which is independent of experience and does not involve interpretation of a nervous signal by the brain. Examples are the shutting of the eye when something comes rapidly towards it and the rapid removal of a hand from a hot object. The response is immediate. When the sense organ is stimulated signals pass from it along a sensory nerve to the spinal cord. The message travels out of the spinal cord along a motor nerve to the effector organ (e.g. a muscle or gland) which acts accordingly. Such a pathway is termed a reffex arc. Sensory fibres enter the dorsal part of the spinal cord while motor fibres leave the ventral part.

Regeneration- The ability to replace parts of the body that have been lost through accident. All animal. and plants are capable of it to some extent but. as a general rule. the more highly evolved an animal is, the less are its powers of regeneration. Man, for example, is able to regenerate skin and bone tissue to mend wounds and fractures, but is unable to regrow even a finger if one should be lost.

Some internal organs, such as the liver, can be regenerated if a large enough part remains as a starting point. The replacement of worn out tissues is a form of regeneration and geos on throughout life. The most obvious is the replacement of skin from below as the outer layers rub off.

Some animals arc able to regenerate new limbs-in fact lizards may actually shed their tails themselves to confuse enemies and later they grow new ones. Crabs, too, can throw off a limb to escape from a enemy. This process is called autotomy. The most striking case of regeneration, however, are found among the lower animals.

When a planarian is cut in half, both halves can form new animals. The cells of the damaged region lose their individuality and all become alike. They grow and divide and the growing mass takes on the correct shape be it of a head or a tail. Gradually the cells become specialised again and begin their normal functions. The head region is the co ordinating region and, if missing, is always the first formed structure in a regenerating body. Not until there is a head, can the other organs be reformed.

Flatworms are built up of three basic layers; an external ectoderm, an endoderm lining the gut, and a mesoderm occupying the space between the two. Planarians are flatworms with great powers of regeneration. As long as all three types of tissue are present and as long as there is sufficient food reserve in the tissue, a complete new planarian can grow from a tiny portion. The fragment has no mouth and feeding organs and so cannot get any food from outside. The cells become alike and multiply. Because food is used up during this process. the fragment gets smaller, but if sufficient cells can be formed, a tiny planarian will develop. Any original organs remaining in the fragment will be partly absorbed so that they conform in size to the rest of the new animal. A new head is formed before the rest of the body develops.

Any fragment of a planarian that regenerates naturally will produce a head at the original front end. Although the cells are completely reorganised they never lose this polarity. Regeneration in jelly fish is very similar, but in higher animals the head cannot be regenerated. New limbs and bodies can form under the influence of the head but not vice versa.

Reproduction- Reproduction is the most important function of any living species; failure to reproduce mean that the species will become extinct. In response to this biological necessity a number of methods of reproduction have developed.

Amongst higher plants and animals sexual reproduction is the most wide spread and successful method. At some stage, specialised reproductive cells (or gametes) develop. One gamete must combine with another to form what is called a zygote. The process of combination is called fertilisation.

Gametes must not only be capable of coming together. Between them, they must provide enough food for the new individual to develop. Usually there is a division of labour; one type of gamete-the female cell or egg-remains stationary but contains a large food supply. The other type the male or spermatozoan carries a minimum supply of food but is mobile and moves towards the egg.

Most commonly, male and female gametes are borne on different animals, but in some species. both are carried by one animal (See Hermaphrodite). Gametes differ from body (somatic). cells in the number of chromosomes in the cells. Body cells have paired sets of chromosomes (diploid condition) but when sex cells are formed the pairs split up (See Meiosis) and the sex cells carry only single sets of chromosomes. For example. the body cells of man have 23 pairs of chromosomes, but the gametes have only 23 single ones. At fertilisation the gametes fuse and form a zygote, divides by mitosis to form a]! the body cells which thus have 23 pairs of chromosomes.

A few animals reproduce by parthenogenesis or by asexual methods. In the latter there are no special cells-pieces of the parent break away and grow into new individuals.

The great advantage of sexual reproduction is that recombinations of genes are continually occurring. At each fertilisation different genetical material comes together. The variety of animals likely to be produced is thereby increased for it is the genes that largely control development. It follows that the chance of an animal appearing with advantageous adaptions is also increased.

With parthenogenesis, and asexual reproduction. however, the off spring will resemble the parent. for exactly the same genes will be inherited. No variations occur. But these methods do have their advantages. Unlike sexual reproduction huge numbers of off spring can be produced in a very short space of time. Also a single individual may colonize a whole new area, a feat which is usually impossible by the sexual method.

Though it is common to find animals and plants reproducing solely by the sexual method. it is very rare to find them breeding solely by parthenogenesis or asexual methods. In these instances the sexual and nonsexual methods alternate at more or less regular intervals (heterogamy). The advantages of both mechainsms are thus obtained.

Reptilia. A class of vertebrate animals that evolved many millions of years ago from one branch of the Amphibia (q.v.). Apart from a few that give birth to active young. the reptiles lay shelled eggs and are thus not dependent on the water for breeding. There is no free larval stage corresponding to the the amphibian tadpole. In many ways reptiles are intermediate between amphibians on the one hand and the mammals and birds on the other. Mammals and birds arose from two different lines of reptiles. Only four relatively small groups of reptiles are alive now but the reptiles once ruled the earth with the huge dinosaurs on land. the flying pterodactyls and the aquatic ichthyosaurs.

Reptiles are covered with scales (from which it is thought birds feathers are evolved) and when limbs are present, they have the basic pentadactyl pattern of five toes. Lungs are the sole respiratory organs. All reptiles are poikilothermic, that is they cannot maintain a constant body temperature and it varies with that of the surroundings. Reptiles are active only in warm weather.

There are four sub classes of reptiles. divided according to the structure of the skull. Sub class Anapsida contains the earliest reptiles such as Seymouria and suprisingly enough also has living members the tortoises (order Chelonia). These have probably survived by virtue of their protective shells. The sub class Parapsida contains only extinct reptiles the ichthyosaurs. Plesiosaurs, and certain other forms. It is probably a polyphyletic group whole members are not closely releated.

The such class Diapsida is the largest of all. Living members include the snakes and lizards order Squamata, the tuatara, and the crocodiles. The dinosaurs and the pterodactyls also belong to this sub class. Sub class Synapsida contains only extinct forms but this line is believed to have given rise to the mammals.

Respiration - All living processes require energy. This is obtained by oxidation of food materials within the body tissues. In all animals and in the majority of plants the process depends on free oxygen absorbed from the surroundings. The absorption of oxygen, its transport to the tissues, and the oxidation reactions are all classed as respiratory activities, but the term 'respiration' is normally confined to the chemical reactions within the cells.

Protozoans and other simple animals get sufficient oxygen by simple diffiusion from the surroundings. Earthworms absorb their oxygen requirements through the skin which is very well supplied with blood vessels.. This arrangement is possible only in damp surroundings. In dry air, water would very rapidly be lost through the skin. All larger or more active animals have special respiratory organs to absorb oxygen. Animals living in water usually have special respiratory organs to absorb oxygen. Animals living in water usually have gills (q.v.) to absorb oxygen from the water. Land vertebrates usually breathe (i.e. take in air) by means of lungs (q.v.) Insects and some other arthropods have a network of tracheae tiny tubes that conduct air from the .surroundings to every part of the body.

The essential features of respiraory organs are: a large moist surface area, very thin walls, and a good blood supply. The gills of fishes and of crustaceans consist of very thin plates of tissue over which water is caused to pass by various movements of the animal. (See Respiratory Movement). Oxygen, dissolved in the water passes into the blood stream and is transported to the tissues. Because a stream of water flows over the gills there is always a fresh source of oxygen next to the gill. Lungs are internal chambers with no Continuous air now. They have to be filled and emptied by breathing movements. Absorption of oxygen into the blood follows the same pattern in lungs and in gills. In both cases the oxygen is in solution when it reaches the respiratory surface.

The blood arriving here is low in oxygen content and thus the oxygen outside passes in solution through the thin walls and into the blood which transports it to the tissues. Only a very small amount of the oxygen is carried as a simple solution in the blood. Most of it combines with a respiratory pigment (q.v.) in the blood. The compound formed is unstable and later releases oxygen in the capillaries of the body. In the body the tissues are low in oxygen content so that the oxygen released passes through the fine capillary walls and into the cells where the chemical reactions take place. These reactions are very complicated and involve numerous enzymes and intermediate stages. The net result, however, can be shown as:
food + oxygen = carbon dioxide + water + energy.

This equation holds good for both plants and animals. Glucose is a commonly used food material. Its oxidation can be shown chemically as follow:

Most of the carbon dioxide released is removed by the blood stream. Some is carried in solution and some in combination with blood proteins, but by far the largest' amount is carried in the form of bicarbonate ions. In the blood capillaries of the tissues carbon dioxide and water combine to form carbonic acid:
CO₂ + H₂O = H₂CO₃

This then breaks down into ions:
H₂CO₃ = H + HCO₃

At the respiratory surface the blood becomes more acid and the bicarbonates are broken down, real easing carbon dioxide which passes through the respiratory surface and out to the surroundings.