Teleostei - Order of bony fishes (Actinopterygii) including about 20,000 known species the vast majority of living fishes. Teleosts seem to have arisen during Triassic times and have since almost replaced all earlier bony fishes. There has been a tendency to a reduction of scales and a shortening of the body. The more primitive teleosts herrings, salmon and others have soft fin rays and retain the duct between the gut and the swim-bladder. The more advanced members of the order perch, cod, and the various flat fishes-have lost the connection between gut and swim bladder and the fin rays are stiff. There is a wide range of form among teleosts with the sea horses, angler fish, flatfish, and eels all included with the more typically shaped fishes.

Telson - The last segment of the abdomen of arthropods. it does not always develop insects do not have one for example but the scorpion sting and the fan shaped tail of lobsters are examples of a well developed telson.

Temperature Control - (See Skin).

Tendon - bundle of collagen fibres connecting muscles to bones.

Tergum - The thickened dorsal region of the arthropod cuticle.

Territory - An thickened dorsal region of the arthropod cuticle.

Territory - An area inhabited or dominated by an animal or a family of animals, especially for the purpose of reproduction. Territory is marked out by scent, or by singing in the case of birds, and intruders of the same species are attacked. Territory is possibly a mechanism of population control for it prevents too many animals from breeding in one area.

Tertiary Period - Division of Geological Time Scale.

Testis - Male gonad producing sperms and various sex hormones.

Testosterone - Vertebrate male hormone. It is responsible for the development of many male characteristics.

Tetrapoda - Term embracing all the basically four footed animals i.e.all the land living vertebrates.

Therapsida - Extinct order of mammallike reptiles (sub class Synapsida).

Thorax - (1) Region of the vertebrate boty surrounded by ribs (when present) and enclosing the heart and lungs. (2) Central region of the insect body consisting of three segments and normally carrying three pairs of legs and two pairs of wings.

Threshold - The level of intensity of a stimulus below which it does not evoke a response from a sense organ.

Thymus - Glandular organ in neck or chest region of vertebrates. Its function is not fully understood but it appears to be concerned with sexual development. If the thymus is removed the development of the reproductive system is delayed. After maturity is reached the thymus degenerates.

Thyroid Gland - An important endocrine gland found in the neck region of vertebrates. Its function is to incorporate oidine from the diet into large organic molecules thyroid hormones which act on all other parts of the body. It has an extremely rich blood supply and it is mainly made up of hollow, spherical sacs or follicles whose walls are one cell thick.

The cavity of each follicle is filled with a protein like jelly known as colloid.

The colloid contains proteins, enzymes, the thyroid hormones
produced by the cells of the follicle wall, and the compounds from which they are built up. The amounts of each that are present vary from time to time and from one follicle to another. The colloid is more than a reservoir, therefore. It is somewhat like a wholesale warehouse, always keeping a basic stock of numerous articles, some finished and others ready to be put together, the numbers of each varying as fresh supplies come in and as orders are despatched. A feature better shown in the thyroid than in any other endocrine gland is the storage of hormone outside the cells that produce it.

Present knowledge indicates that the major hormone secreted by the thyroid is thyroxine. Its molecule is formed by the combination of two molecules of tyroxine. Its molecule is formed by the combination of two molecules of tyrosine (an amino acid) each of which has had two hydrogen atoms replaced by two atoms of iodine with the elimination of another amino acid alanine. A substance similar to thyroxine but containing only three iodine atoms is also produced by the thyroid. It is called triiodothyronine and, although produced in smaller amounts than thyroxine, it has a greater effect on the body cells. Both hormones cause an increase in the metabolic rate.

The failure of the thyroid produces both mental and physical retardation. Lack of iodine on which the thyroid can act produces similar effects. Goitre enlargement of the thyroid is also produced by lack of iodine in the diet and it is interesting to note that goitre occurs more commonly in inland and mountainous areas where the iodine content of the soil is low. Administration of iodine or thyroid extract can cure these deformities.

The activities of the thyroid are closely related to those of the tissues as a whole but the relationship is not a simple one. The pituitary gland is particularly important in controlling the thyroid. It produces the thyrotropic hormone which causes the thyroid to elaborate more thyroid hormone. The amount of thyroid hormone in the blood itself affects the production of thyrotropic hormone; a high level reducing its production, a low level increasing it.

Tibia - The shin bone larger of the two bones in the lower hind leg.

Tissue - A patch or group of similar cells all performing the same function, e.g. muscle cells, liver cells, etc.

Toads - (See Anura).

Tonsil - Lymphoid tissue in throat region of tetrapods, concerned with the defence of the throat and mouth region against bacteria.

Tooth - Bony process in the jaws of most vertebrates concerned mainly with the capture and breaking. up of food. The teeth of reptiles and other lower vertebrates are all of a simple conical shape, but mammalian teeth are of several different types, modified according to the diet. At the front of the jaws there are a number of chiselshaped incisor teeth. These are followed by a stabbing or eye tooth the canine and then the grinding cheek teeth-premolars and molars. The majority of mammals have two sets of teeth a first milk set, and a permanent set replacing them later in life. Molar teeth, however, are not preceeded by milk teeth. The precise number, structure and arrangement of teeth differ between animals that have different diets.

In spite of the differences in external form, all teeth are constructed on the same basic plan. Projecting from the gum is the crown of the tooth. The part embedded in the gum and reaching into a socket in the jaw bone is known as the root. The body of the tooth is made up of hard bone like substance known as dentine. Inside this is the pulp cavity which contains blood vessels and nerves. Branches from these, together with fine protoplasmic threads, penetrate the maze of fine canals which spread throughout the dentine. Covering the crown of the tooth is a layer of enamel of varying thickness. It is made up almost entirely of apatite crystals with calcium phosphate tilling. Apatite crystals are made up of calcium phosphate plus calcium fluoride or calcium chloride.

Calcium phosphate is also mainly responsible for the hardness of dentine. The enamel crystals are elongated and are all arranged with their ends towards the surface of the enamel. Around the root of the tooth, enamel is replaced by cementum, another bone-like material which fixes the tooth firmly in the socket of the jaw. However, between the jaw bone and the cementum there is a layer of tissue the periodontal membrane- which is in contact with the tissues of the gums and with the pulp cavity. Incisor and canine teeth have a single root, premolars have a double root and molars have three branches to the root.

Although teeth are composed of hard bone like tissue and are embedded within the jaw bones, they are in fact, derived from the skin in much the same way as the placoid scales of shark like fishes. The similarities of structure and development of these scales and the teeth of vertebrates make it certain that both have evolved from the bony plates that covered the primitive fishes. Tooth formation begins in the early embryo. Buds of both milk and permanent teeth are formed and they develop slowly, the various layers growing until the complete crown is formed within the the jaw bones because the jaw bones grow around the developing teeth in the embryo. When the crown is complete, the tooth erupts through the gum by growth of the root. The permanent teeth develop even more slowly at the base of the milk teeth. Pressure of the developing permanent teeth forces out the milk teeth later in life.

Torsion - Gasteropods with coiled shells have a mantle cavity at the front with the gut positioned behind. The mantle cavity appears behind the gut but during development the position is reversed. This is called torsion

Trachea - (1) Wind pipe of tetrapods, leading from throat to bronchi(See Lung). (2) Breathing tube of Insecta.

Tracheole - The fine ending of tracheae in Insecta.

Tremstoda - Class of Platyhelminthes. All are endoparasitic and covered with a thick, structureless cuticle, sometimes equipped with spines. They have suckers with which they attach themselves to their hosts. The mouth leads to a two branched gut. Some species have only one host but more usually there are two or more hosts involved in the trematode life history and the trematode passes through several larval stages. Examples include Fasciola, the liver fluke and Schistosoma, the cause of Bilharzia in man.

Triassic Period - Division of Geological Time Scale.

Trichoptera - Order of Insecta containing caddis flies. The wings and body of the adult are covered with fine hairs. Metamorphosis is complete and the larvae live in water. The majority of the larvae build cases of small stones, leaves, or other fragments, in which they conceal their soft bodies.