Zoology Terms Starting With G

Gastrolith is a stone swallowed by an animal and retained in its digestive tract to grind food mechanically or, in some aquatic species, to adjust buoyancy.

Gastroliths lodge in the muscular gizzard of birds, crocodilians, and some other animals, where repeated contraction of the digestive tract forces the stones against tough seeds, plant fibers, and hard-shelled prey. This grinding compensates for limited chewing and reduces particle size before enzymatic digestion begins. Aquatic reptiles such as some plesiosaurs show gastrolith accumulations that have also been interpreted as possible ballast or stability aids in the water column.

Sauropod dinosaurs present a more debated case, with many researchers arguing that the stones were too few relative to body size to grind the enormous volumes of plant material these animals consumed.

Gill is a respiratory organ in aquatic animals that extracts dissolved oxygen from water across a thin, vascularized surface and releases carbon dioxide into the surrounding water.

Fish gills consist of gill arches bearing rows of filaments, each covered in microscopic folds called lamellae that expand the gas-exchange surface area to many times the body surface area. Water flows across the lamellae in one direction while blood flows through lamellar capillaries in the opposite direction, a countercurrent arrangement that maintains a favorable diffusion gradient along the entire exchange surface and can extract more than 80 percent of the dissolved oxygen from passing water. Beyond gas exchange, gills regulate ion balance and water movement; in freshwater fish, specialized ionocytes in the gill epithelium actively absorb sodium and chloride ions to compensate for osmotic dilution.

Some amphibians, such as the axolotl (Ambystoma mexicanum), retain external gills throughout their lives, a condition called neoteny.

Gizzard is a thick-walled, muscular region of the digestive tract found in birds, crocodilians, earthworms, and many other invertebrates, that grinds food mechanically through muscular contractions, often assisted by ingested stones called gastroliths.

In birds, the gizzard follows the proventriculus, which handles enzymatic and acid digestion, and consists of two pairs of opposing smooth muscle masses that generate powerful grinding forces against a hardened keratinous lining. This mechanical processing compensates for the absence of teeth, allowing seed-eating species to fracture seed coats that would otherwise resist enzymatic digestion. Earthworms pass soil and organic matter through a gizzard lined with calcium carbonate plates that crush particles before they enter the intestine.

Some sauropod dinosaurs are thought to have used gizzard-like stomach chambers based on polished gastrolith accumulations found in association with their skeletal remains.

Gonad is a primary reproductive organ in animals that produces gametes and secretes steroid hormones regulating sexual development, reproductive cycles, and secondary sex characteristics.

Testes produce sperm through spermatogenesis, generating roughly 1,500 sperm per second in adult human males, while ovaries produce eggs through oogenesis and release them during ovulation. Both organs secrete steroid hormones, with testes producing primarily testosterone and ovaries producing estrogen and progesterone, that travel through the bloodstream to regulate bone density, muscle mass, reproductive behavior, and the timing of puberty. Gonadal hormone output is itself controlled by gonadotropins released from the pituitary gland in response to gonadotropin-releasing hormone from the hypothalamus, forming a feedback loop that adjusts hormone levels across the reproductive cycle.

This dual role in gamete production and hormone secretion makes gonads both reproductive and endocrine organs.