Frog zoology

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[edit] 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 oblongata regulates respiration, digestion, and other automatic functions. Muscular coordination and posture are controlled by the cerebellum. The relative size of the cerebrum of a frog is much smaller than that of a human. 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 smelling and breathing. The nostrils have complex valves, but no long nasal passages. Air flows from the nostrils and into the frog's mouth, then down into the lungs. 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.

[edit] Reproduction

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 semen (which is colored yellow.) 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 develops, 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.

[edit] Circulation

The circulatory system of frogs is consistent in a methodical manner. Oxygen-laden blood is received by the left atrium and then enters the frog single ventricle. 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 left and right ventricle. The oxygen poor blood and the oxygen rich blood 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.

[edit] Locomotion

Frogs have saltatory locomotion. This means that their primary method of locomotion is by hopping, jumping, or leaping. The front arms 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 suited for leaping. These legs contain a tibia, as well as a fibula (shinbones), 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).

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