Frogs are animals that belong to the class Amphibia, commonly known as amphibians. Amphibians live both on land and in the water; sometimes they live in water only before they are fully grown. They have moist skin that water can pass in and out of, and most of them spend part of their life (in the tadpole-like larva stage before they mature) in the water. The name Amphibian means literally 'both life.' There are three orders of amphibians: frogs and toads with over 2,000 species, salamanders (including newts) with approximately 300 species, and caecilians with about 160 species.
What makes frogs different from other amphibians? They have smooth skin, are usually tailless as adults, and have the ability to leap. Some frogs are aquatic, spending most of their time in the water. Others are terrestrial, staying mostly on land or in trees. (Toads are also tailless, leaping amphibians, but they usually have rough, warty skin.) Frogs range in size from less than 0.5 inches to 12 inches long!
Frog Life Cycle
A frog begins life as a fertilized egg. The female frog usually lays the eggs in water in a string or mass that sticks to vegetation. The male frog fertilizes the eggs as they are laid. The outer layer of a fertilized egg is a jelly-like material that swells in water, forming a protective coating. The fertilized egg is a single cell that rapidly divides again and again, producing new cells that quickly differentiate into the organs of the frog embryo. Within 2 to 25 days, depending on water temperature, the egg hatches into a tadpole. The tadpole looks more like a fish at first than like a frog. As the tadpole develops, it forms gills that allow it to breathe efficiently underwater. Its tail grows longer and a fin forms, which allows the tadpole to swim effectively.
The tadpole continues to swim, eat and grow for several weeks before it matures to the next stage. The first sign of further development is the appearance of hind legs. Then front legs develop and the tail becomes shorter as it is resorbed. Internally, the tadpole's gills are replaced with lungs until finally the tadpole has become a frog. The young frog grows and matures to adulthood over a period of 2-4 years. The adult frogs then lay their eggs and begin the cycle again.
Anatomy of Adult Frogs
Some frogs are able to leap 20 times their body length! Their front legs are short and specially designed to absorb the impact of landing. Their muscular back legs also work well for swimming. Aquatic frogs have webbed rear feet, usually with five toes. Their front feet are not webbed and usually have four toes. Tree frogs have suction cups on their toes that allow them to cling to the bark of trees.
Frogs have large, bulging eyes that rotate in their socket, providing sight in almost any direction. Their nostrils are located on the top of the head to allow breathing while most of the head is submerged. Although frogs have a good sense of hearing, they don't have typical external ears. Instead, frogs have a tympanic membrane behind each eye. These pick up sound waves and carry them into their internal ears. Frogs' tongues are usually long and sticky and designed to be flicked out quickly to catch insects and other prey.
Frogs have skin that is specially designed to protect them from their enemies and to protect them from drying out. To hide from their enemies, frogs have camouflage skin colorings that help them to blend in with their surroundings. Special pigment cells in their skin control the camouflage pattern and colors. Also, some frogs have serous glands in their skin, which secrete poison that will irritate the mouth of their predators. South American tree frogs secrete a deadly poison, but most are just irritating to humans. To help them keep from drying out, frogs have mucous glands that secrete a waterproof coating to keep their skin moist and slippery.
Have you ever wondered how frogs breathe? When under water, frogs get their oxygen from water that passes through their skin. Capillaries take the oxygen from the skin into the bloodstream. On land, frogs usually get oxygen by taking air through their throats into saclike lungs. Frog hearts have three chambers. If you really want to see frog anatomy for yourself, we recommend doing a frog dissection.
To find out more about frogs, do research on one of these topics: what kinds of frogs live in your area? Can you find more than one species of tadpole locally? If so, compare them. What do local frogs eat? How would the mosquito population be affected if there were few or no frogs in a swampy region? Pick a frog or frog characteristic that is interesting to you, and see what you can find out about it. Look for close-up frog pictures in a magazine like National Geographic or on a website.