Gill slit. An opening leading from the pharynx to the exterior in aquatic vertebrates and Amphioxus. In Amphioxus they function in filter feeding. In fish they contain the gills and are usually in the form of serried of long slits. They are absent in adult tetrapod vertebrates (except for some amphibians) but their presence in some form in the embryos of all vertebrates is a characteristic of the phylum Chordata.

Girdle. A bony structure to which the limbs of vertebrates are attached.

Gizzard. Part of alimentary canal in many animals, where food is broken up. Percedes main digestion region. Very muscular walls; often contains hard ‘teeth’ attached to walls, e.g. Crustacea, or hard swallowed objects, as in birds.

Glaciation. One of the so-called ice ages; a period of ice cover.

Gland: A group of cells or a single cell in animals or plant that is specialized to secrete a specific substance. In animals there are two types of glands, both of which synthesize their secretions. Endocrine glands discharge their products directly into the blood vessels, exocrine glands secrete through a duct or network of ducts into a body cavity or onto the body surface.

Glia. The web of undifferentiated cells that packs and supports nerve cells in the brain and spinal cord.

Globigerina ooze. Calcarerous mud, covering huge areas of ocean bottom (about 1/3 of whole sea floor) mainly formed from shells of Foraminifera (Protozoa), Globigerina being important genus.

Globulin. Protein that is without a prosthetic group and is soluble in dilute salt solution but not water. (Cf. Albumin) Widely distributed in cells of plants and animals. Amongst globulins are the main proteins of plant seeds; and antibodies.

Glochidium. A lamellibranch larva possessing a small bivalve shell and a tentacle-like sucker for attachment to a fish.

Glomerular filtrate. A filtrate that has passed through the Glomerulus in the vertebrate kidney.