Frog: excretion, coordination and reproduction

Excretion. A frog has a pair of dark red, bean-shaped kidneys, made of many tiny tubes (nephrons, or uriniferous tubules). Urine flows through the ureters to the urinary bladder and leaves through the cloaca. Frogs get rid of nitrogen waste mainly as urea: they are ureotelic. In males, the ureters also carry sperm, so they are urinogenital ducts.

Hormones come from endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. The nervous system has three parts: central (the brain, inside a bony box, the cranium, and the spinal cord), peripheral (ten pairs of cranial nerves and the spinal nerves), and autonomic.

The brain has three regions. The fore-brain: olfactory lobes (smell), cerebral hemispheres and diencephalon. The mid-brain: a pair of optic lobes (sight). The hind-brain: cerebellum and medulla oblongata. Sense organs: sensory papillae (touch), taste buds, the nasal lining (smell), simple eyes, and the tympanum with internal ears, which handle both hearing and balance.

Male: a pair of yellowish testes held on the upper part of the kidneys by a fold called the mesorchium. Ten to twelve fine tubes (vasa efferentia) carry sperm into the kidney, to Bidder's canal, and out through the urinogenital duct. Female: a pair of ovaries with no connection to the kidneys; the oviducts open separately into the cloaca. A female lays 2,500 to 3,000 eggs at a time.

Fertilisation is external, in water. The egg hatches into a fish-like larva, the tadpole, which changes shape completely (metamorphosis) into a young frog. Frogs are useful: they eat insects and so protect crops, and they are important links in food chains.