Gastric - Concerning the stomach.

Gene - An hereditary factor passed all tram cell to cell and from generation to generation, which has a particular effect on the cell or organism containing it. Genes are carried on the chromosomes (q.v.), in the cell nuclei and each gene has a particular place on a certain chromosome. Because each body cell has two sets of chromosomes, it has two genes concerned with each feature. But the two genes are not feature. But the two genes are not necessarily identical; every now and then the molecular structure of a gene may become alteed so that, although it still affects the same feature of the organism, it has a different effect. The gene is said to have mulated. A pair of different genes which occur at the same place on the chromosmes but have different effects on the development of the organism are said to be alleles or allelomorphic genes. As a rule, one allele over rules the effect of the other and is said to be dominant. The other allele is the recessive and its presence is made. apparent in the organism only if it is present in both sets of chromosomes. If the two chromosomes carry identical genes, the organism is said to be homozygous for that character. If two different alleles are carried, the organism is heterozygous.

Genes are believed to be composed. of nucliec acids which control the development of the cells by controlling the types of protein formed. The vast number of instruction carried by the genes of an organism is made possible by the enormous variation in the arrangement of the molecules make up the nucleic acids. When a cells divides (except when forming sex cells) the chromosomes and genes duplicate themselves exactly so that the same instructions go to each new cell. But even then the new cells may not be exactly like the parent for their position in the body also influences their development. When sex cells are formed the chromosomes do not duplicate themselves but one member of each pair goes to each sex cell. When sex cells join at fertilisation, there are again two sets or chromosomes but the genes that they carry are not identical with those of either parent and, as the effect of a gene is modified by the action of those genes around it, the new generation will not be identical with either parent.

Genetics - The study of genes and the way in which characteristics are passed on to offspring.

Genotype - The actual genetic constitution of an organism, which may not be apparent from the outward appearance.

Genus - A category used in classification, consisting of a number of closely related species, all of which share the generic name.

Geological - Time Scale. During the Earth's long history there have been many changes: land has been submerged and uplifted many times; animal groups have appeared, flourished, and become extinct. Geological time has been divided into a number of periods separated by episodes of mountain building or abrupt faunal changes. The times scale is of great importance to zoologists dealing with evolution and with fossil groups.

Germ Cell - A gamete or sex cell.

Gestation Period - Period from fertilisation to birth in mammals. It varies from about three weeks in house mice, to about 40 weeks in man and nearly two years in elephants.

Gill - Respiratory organ of various aquatic animals. Consists of thin plates of tissue, well supplied with blood. Gills may be inside or outside the body but they are open to the water from which they extract oxygen for respiration.

Gill Book - Respiratory organ of certain aquatic arachnids, consisting of plates of tissue arranged somewhat like the leaves of a book.

Gill Slit - An opening from the pharynx to the outside of the body. Gill slits are diagnostic characterstics of chordate animals, although they are found only in the embryos and young stages of tetrapods. In Amphioxus and the sea squirts there are many gill slits and they are concerned with filtering food particles from the water. In fishes there are fewer slits and they are modified for respiration.

Gizzard - Muscular region of the alimentary canal of certain animals where food is ground up prior to main digestive processes.

Gland - An organ that manufactures certain substances and passes them out into the body where they fulfil particular functions. Digestive juices are produced in glands such as the liver and pancreas. These have ducts to carry away the secretions and are called exocrine glands. There are, however, several ductless or endocrine glands, such as the thyroid and adrenals. These produce hormones which are carried to their sites of action by the blood.

Glenoid Cavity - Cup shaped depression in shoulder girdle into which head of humerus tits.

Glomerulus - Knot of blood capillaries in Bowman's capsule of kidney.

Glossopharyngeal nerve - Ninth cranial nerve, supplying throat region.

Glottis - Opening of the windpipe (trachea) into the pharynx.

Gluteal Muscles - Muscles of the buttocks, concerned with erect posture and therefore small in quadrupeds.

Glyeogen - 'Aninmal starch' a polysaccharide that is stored by various animals in their bodies and which can be converted into glucose when required for respiration.

Gnatbostomata - Term sometimes used in classification to refer to all the jawed vertebrates

Golgi Apparatus - (See Cell).

Gonad - Orgar here sex cells are formed i.e.testis ovary.

Granflan Follete - Structure in mammalian ovary in which an actual egg cell develops and which bursts to release egg cell at ovulation.Graptolite Extinct animal whose relationship to others groups is not clear. Graptoli are abundant in palaezoic rocks and they are of great importance to geologists for correlaing outcrops in different parts of the worlds. The fossils are normally found is white markings on slaty rocks. There are many species but all appear to have been colonial the individual animals living in tiny cups on branched or unbranched stalks.

Green Gland - Excretroy organ of crustaceans.

Grey Matter - That part of the vertebrate central nervous system that contains most of the actual nerve cell bodies. It makes up the central part of the spinal cord and also the cerebral cortex of the brain. There are vast numbers of nerve connections (synapses) in the grey matter and it is the seat of co ordination.

Gynandromorph - An animal in which, through disturbance of the sexcontrolling mechanism during embroyonic development, part of the body is genetically male and part genetically female. The phenomenon is quite common in insects and may result in one side of the insect having male features and the other side female features.