Frog zoology
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Introduction to phylum
About 400 million years ago, some members of the sarcopterygian group of fish moved onto land. These became the first amphibians. Today, these animals still spend parts of their lives in water and return to water for reproduction. There are about 3,000 known species of modern amphibians, which includes Anura (frogs and toads), Urodela (newts and salamanders), and Gymnophiona (burrowing amphibians). The word amphibian means "both sides of life," coming from the Greek amphi, meaning both, and bios, meaning life.
Digestion and Excretion
The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called the maxillary teeth, which are used to grind food before swllowing. These teeth are very weak, and cannot be used to catch or harm agile prey. Instead, the frog uses its sticky tongue to catch food (such as flies or other insects). The food then moves through the esophagus into the stomach. After being broken down with stomach acid, the food then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Ducts carry pancreatic juice from the pancreas and bile (produced by the liver) through the gallbladder from the liver to the small intestine, where the fluids digest the food and extract the nutrients. When the food passes into the large intestine, the water is reabsorbed and wastes are divided into liquids and solids. Liquid wastes go to the urinary bladder, while solids are routed to the cloaca. All wastes exit the body through the cloaca and the cloacal vent.
Nervous System
The frog has a highly developed nervous system which consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. The medulla regulates respiration, digestion, and other automatic functions. Muscular coordination and posture are controlled by the cerebellum. A frog has a very small cerebrum; that of a man is much larger (relatively). In humans, the cerebrum runs many important life processes.
Frogs have ten cranial nerves (nerves which pass information from the outside directly to the brain) and ten pairs of spinal nerves (nerves which pass information from extremities to the brain through the spinal cord). By contrast, humans have twelve cranial nerves and 30 pairs of spinal nerves.
The frog has crude eyes and two nostrils for smell. The nostrils have complex valves, but no long nasal passages. Frogs do not have external ears; the eardrums (tympanic membranes) are directly exposed. As in humans, the ear contains circular passages which help control balance and orientation.
Reproduction
Frogs use external fertilization in their reproductive cycles. The female frog begins by laying a clutch of eggs. This can be very large, depending on the type of frog. The male then sheds sperm over the eggs to fertilize them. Eggs are almost always laid in a body of water, although some frogs will lay eggs on vegetation which hangs over a river or other body of water. This allows the tadpoles to fall from the vegetation into the water after hatching.
The length of time necessary for frog eggs to hatch (the embryonic stage) varies depending on the size of the eggs and the environment in which they have been laid. Frog eggs laid in warm water may develop in as little as one to two days, while those deposited in a cold mountain stream could take 30 to 40 days.
The embryonic frog hatches into a larval stage, commonly referred to as a tadpole. Frog larvae have short, ovalloid bodies and long tails. The larvae take in water through the mouth, where it circulates through the tadpole's internal gills to be expelled from one of the openings on the side of the tadpole's head.
After the larval stage, the frog goes into metamorphosis. During the metamorphosis, the tadpole grows two pairs of legs and its tail is absorbed. The gills are also absorbed as the gill slits close and lungs develop. The frog's skin becomes thicker, eyelids and a tongue develop, and various bones and teeth grow. The frog's intestine becomes shorter and the frog develops a new system of kidneys, various sensory glands, and gonads.
Circulation
Frogs have three-chambered hearts. Like humans, frogs possess a left and right atrium; however, frogs have a single ventricle, while humans have two. The ventricle always contains both oxygen-rich blood and oxygen-poor blood. They do not mix because the right atrium dips downward into the ventricle, causing the oxygen-poor blood entering the right atrium to pass to the bottom of the ventricle. The oxygen-poor blood fills the lung vessels and forces the incoming oxygen-rich blood to detour into the arteries, where it carries oxygen and nutrients to the tissues. Frog blood is made up of a liquid plasma which carries solid elements such as red and white blood cells.
Respiration
A characteristic of amphibians is the flexibility of their respiration systems. The skin of amphibians is moistened by secretions of mucous, and is used for respiration at times. There are a number of blood vessels near the skin. When a frog is underwater, oxygen is transmitted through the skin directly into the bloodstream. Adult frogs use their lungs when on land. Their lungs are similar to those of humans, but the chest muscles are not involved in respiration and there are no ribs or diaphragm to support breathing. Frogs can breathe by simply opening their mouth and letting air flow into their windpipe. They can also breathe with their mouths closed by taking air in through the nostrils (causing the throat to puff out), and then compressing the floor of the mouth, which forces the air into the lungs.
Locomotion
Frogs have saltatory locomotion. This means that their primary method of locomotion is by hopping, jumping, or leaping. The front legs of a frog are similar to the arms of humans. They are weak and of little use for jumping. However, the frog has hind legs which are especially designed for leaping. These legs contain a tibiofibula (shinbone), femur, and the astragalus and calcaneus (similar to a human's ankle and heel bones). Adult frogs move about on land by hopping with their powerful hind legs.
Tadpoles, which have not yet developed arms or legs, use their tails to undulate like a fish (such as the perch).
Symmetry
Many higher-class animals (including perch, rats, and humans) feature bilateral symmetry. All amphibians are bilaterally symmetrical.
Special characteristics
All frogs have poison glands in their skin. Some frogs, such as the arrow-poison frog, are especially toxic. The chemical makeup of these toxins varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors (which act to narrow the blood vessels). However, these toxins do not provide security from many predators. Edible frogs rely on skin modifications for protection. The more poisonous frogs feature bright colors to warn off predators. There is also one nonpoisonous frog in South America which mimics a poisonous frog?s colors to protect itself. Other frogs use camouflage to blend with dead leaves, lichens, and other environmental elements. Certain frogs change color between night and day, which is caused by light and moisture stimulating the pigment cells and causing them to expand or contract.