The sting is rarely used for capturing food unless the latter is large, but some species use it more readily than others. It appears to be mainly a defensive weapon. The strength and effect of the scorpion's poison varies from one species to another. Some have only a weak poison that does not seriously harm Man. Its effects are confined to the tissues in the neighbourhood of the wound. Others, however producing convulsions and sometimes death.

Scrotum - Pocket of skin containing the testes of most adult mammals. Normal body temperature inhibits sperm production but the temperature in the scrotum is slightly lower.

Scyphozoa - Class of Coelenterata in which the main stage of the life history is the medusa or jellyfish. In some species there is also a polypstage which gives rise to the medusae by budding. Some of the jellyfishes are very large.

Sea Cucumber - (See Echinodermata).

Secretia - A hormone which is produced by the wall of the first part of the small intestine in response to the acid chyme passing from the stomach. Secretin passes into the blood and promotes the flow of bile and pancreatic juice into the intestine.

Segmentation - The repetition of a number of basically similar sections or segments along an animal's body. Not all animals are segmented molluces, for example, show no trace of segmentation some animals such as the annelids and arthropods whow marked segmentation; in others, such as the vertebrates, it is less obvious being seen mainly in the structure of the vertebral column and in the repetitive arrangement of the spinal, nerves.

Semen - Material produced by male reproductive organs, consisting of sperm and various other secretions from the accessory glands.

Semicircular Canals - Organs of balance in vertebrate ear.

Seminal Vesicle - (=Vesicula seminalis).

Seminiferous Tubules - Coiled tubules in vertebrate testis in which sperms are actually produced.

Sense Organ - (=Receptor).

Sensory - Concerned with receiving stimuli and transmitting then to central nervous system.

Septum - Partition, especially between segments in worms.

Sessile - Fixed to the substrate, e.g. sea anemone. Also means unstalked.

Sex Chromosomes - The Chromosomes that determine sex. Although the body cells normally contain pairs of similar chromosomes, one sex (the male in mammals, female in birds and certain insects) contains a pair of dissimilar chromosomes called X and Y. These are the sex chromosomes. The opposite sex contains two X chromosomes and is called the homogametic sex. The homogametic sex produces only gan1etes with X chromosomes but the heterogametic sex produces equal numbers of X and Y gametes. When gametes join, they thus produce more or less equal numbers of each sex in the offspring.

Sex Lindage - The Y chromosome (See Sex Chromosomes) rarely carries genes other than those concerned with sex, but the X chromosome often does. These other genes .are said to be sex linked. Recessive genes on the X-chromosome will always show up on the male or heterogametic sex because there is no gene on the Y chromosome to dominate it. Conseqently men show far more recessive characteristics than women for, in females, the recessive genes are more than usually over ruled.

Sexual Selection - A possible evolutionary mechanism suggested by Darwin. It assumes that females select those males which have the most attractive (to the female) features. This could explain the evolution of the bright colours and mating behaviour of many animals but, because animals normally exist with a balance between the sexes, and most individuals will therefore be able to reproduce, it is unlikely that sexual selection has been of much importance in evolution.

Sharks - (See Elasmo branchii)

Shell - Loosely applied to any hard external skeleton but properly restricted to the hard covering of molluscs whether a single valve as in snails or two valves as in mussels. Basically the shell is made up of three layers, an outer horn like layer called the periostracum, a middle prismatic layer and an inner nacreous layer. The periostracum is made of a substance called conchiolin which has a very similar chemical structure to chitin, an important part of the arthropod shell. The prismatic layer consists of prisms of calcium carbonate arranged at right angles to the periostracum and to the nacreous layer.

Each prism has many layered column of conchiolin separating it from its neighbour. The nacreous layer is made of alternate layers of nacre or 'mother of pearl' (a form of calcium carbonate) and conchiolin. Whereas the prismatic layer and the periostracum are formed at the edge of the mantle, the nacreous layer is formed from the whole of the mantle and is released from its outer surface.

Silurian Period - Division of Geological Time Scale.

Sinus - A space within certain bones of the face, connecting with the nasal cavity. Also an expanded vein of a type found especially in shark like fishes. The sinus venosus is the expanded, thin walled chamber in lower vertebrates (not birds and mammals) where blood collects before entering the heart.

Sinus Venosus - (See Heart)

Siphon - Feeding and breathing tube of vertain molluscs, formed from folds of the mantle. (See Lamellibranchiata ).

Siphonaptera - Order of Insecta containing the fleas. These insects are secondarily wingless-an adaptation to their parasitic mode of life among the hairs of the host and are slightly flattened from side to side. The mouthparts are adapted for blood sucking.

Sirenia - Order of mammals which are completely aquatic the dugongs and manatees, also known as sea cows.

Skeleton - A feature of most animals that gives shape and rigidity to the body, protects soft parts, and provides anchorage for the muscles. The skeleton is normally quite hard and may be inside (endoskeleton) or outside the body (exoskeleton). Exoskeletons include the shells of most molluscs and the hard coverings of crabs and other arthropods. Endoskeletons are found mainly in vertebrate animals and are normally composed of a series of bones, each articulating with its neighbours to allow movement of the body.

Skin - The covering of the body, but it is more than just a covering, it is an important organ in its own right and performs a number of vital functions. This brief account deals mainly with the mammalian skin.

There are two distinct regions in the skin the outer epidermis and the dermis underneath it. The innermost layer of the epidermis is the Malpighian layer. Its cells are living and divide quite frequently in a plane parallel to the skin surface. This layer contains pigment and is responsible for the colour of the skin. As the cells of the Malpighian layer divide, the outer ones are pushed towards the surface and become flattened. They gradually lose all their protoplasm and end up as horny scales which flake off as scurf. On some parts of the body, however, these cells build up thick horny layers (e.g. the sole of the foot). Hair, nails and feathers are all outgrowths of the epidermis.

Underneath the Malpighian layer is the dermis. It is composed largely of connective tissue and, unlike the epidermis, contains many nerves and blood vessels. Elastic fibres in the dermis give skin its elasticity but in old age, the skin loses this property and wrinkles develop.One of the most obvious functions of the skin is that of protection. Its elasticity is some protection against mechanical damage and its waterproof quality prevents excessive water loss. The skin also prevents the entry of germs which could harm the body tissues.

Mammals are warm blooded creatures and can keep their body temperature more or less constant. The skin plays a large part in this temperature control. If, for any reason such as vigorous exercise or fever, the body temperature rises, the tempertature of the blood reaching the brain will be higher. Nerve impulses are then sent out to the blood vessels and muscles of the skin. The vessels widen and carry more blood close to the skin surface and the blood loses heat to the air.When the body temperature falls, several changes occur. The chemical reactions within the body are usually speeded up to produce more heat energy. Shivering may occur-this is a subconscious act that, through muscular action, produces warmth. Mammals are able to fluff up their hairs by contraction of the muscles attached to the bases of the hairs. This traps a thicker layer of air around the body and reduces heat loss. Goosepimples in Man are produced by contraction of the same muscles.

The blood vessels of the skin contract when the body temperature falls and less heat is lost to the air. The skin is then pale for less blood is flowing near the surface.Sweating is another very important temperature control mechanism, especially in Man. Sweat glands are tiny coiled tubes lying in the dermis and opening onto the skin surface. The human body has perhaps three million of them scattered over the surface. Water and various salts accumulate in the tube and pass out onto the skin surface where the water evaporates.

As a rule, it evaporates immediately and is unnoticed but the total cooling effect of the evaporation is quite considerable. In a humid climate, or during exercise, sweat may be unable to evaporate fast enough and beads of perspiration form on the skin. Sweating is controlled by the nerves and in cold weather the rate is reduced. The water and other waste materials still have to be removed however, and this explains the greater number of visits to the lavatory in cold weather. The salts passed out with sweat are not all waste and include some sodium chloride. The salt content of the diet should be increased in hot climates. Sweating is not effective in those mammals that are clothed with dense hair. In hot weather many hang their tongues out and lose water and heat from their mouths.