Amphibia-Frogs & Their Relatives
From an evolutionary perspective, amphibians are traditional animals, living first in water and then on land during specific times in the life cycle. They are equipped for this double life, so to speak, by body structures and organs that change to meet their needs.
An amphibian must spend part of its life cycle in the water where its eggs are laid and fertilized. The eggs develop into a larval stage or tadpole, that has fishlike characteristics. In tadpoles breathing is through gills, blood is pumped by a two-chambered heart, and swimming is using a tail and body movements made possible by muscles in the body wall. The change to adult form is known as metamorphosis, a process controlled by the thyroid gland. The adult amphibian loses the gills, lateral line senses, tail, unpaired fins, and muscles controlling them - the fish characteristics - and develops structures adapted for life on land. An adult amphibian breathes using lungs and has a three-chambered heart which is more efficient at pumping blood between the lungs, the heart, and the rest of the body. In most species the adult also has limbs for movement, but no tail.
There are 3 general types of amphibians.
1. The Apoda are worm-like, legless, ground-burrowing forms found in tropical and semitropical regions. Caecilia is a representive genus.
2. The Urodela are amphibians that do not lose their tadpole-like tail in metamorphosis; salamanders, newts, and mudpuppies are examples.
3. The Anura, represented by frogs and toads, lose their tails on becoming adults. Frogs and toads are the first vertebrates to become vocal.
Fig: Types of Frog.
Tadpoles have gills that they use to breathe underwater. However, the gills are underdeveloped, and the tadpoles need to take in small amounts of air. A tadpole rises to the surface of the water and opens its mouth to suck in an air bubble. When it closes its mouth, the bubble bursts and sends oxygen to the lungs.
The adult frog also breathes in two ways, but it breathes through its skin and with its lungs. The skin does not merely protect the frog but also helps in respiration.
The frog is covered by soft, thin, moist skin composed of two layers, an outer epidermis and an inner dermis. An extensive network of blood vessels runs throughout the skin. Oxygen can pass through the membranous skin, thereby entering directly into the blood. When a frog submerges beneath the water, all its respiration takes place through the skin. Oxygen is obtained directly from the water.
Adult frogs have paired, simple, saclike lungs. As in humans, air enters the body through two nostrils, passes through the windpipe, and is received by the lungs. The mechanism of breathing, however, is different in the frog from that in humans. In humans, breathing is aided by the ribs, the diaphragm, and the chest muscles. The frog has no ribs or diaphragm, and its chest muscles are not involved in breathing.
A frog may breathe by simply opening its mouth and letting air flow into the windpipe. However, it may also breathe with its mouth closed. The floor of the mouth is lowered, causing the frog’s throat to puff out. When the nostrils open, air enters the enlarged mouth. Then, with nostrils closed, the air in the mouth is forced into the lungs by contraction of the floor of the mouth.
Fig: Necturus, the mud puppy.
In frogs, fertilization is external. Sperm leaves the testes through tubules called vasa efferentia which communicate with the kidney. The sperm cells then pass into the Wolffian duct which leads to the cloaca, a passageway that opens to the outside of the body. In the female large egg masses are released into the body cavity from two ovaries, located at the anterior end of each kidney. Beating cilia sweep the eggs into coiled tubules known as oviducts where they are propelled to the cloaca and then out of the body. As the eggs pass through the oviducts they are coated with a thin layer of jelly-like material. At the time when the female is depositing eggs in the shallow waters of a pond or brook, the male deposits sperm swim to the eggs; then, as each sperm reaches an egg, it digests its way through the jelly and into the eggs; effecting fertilization. After fertilization, the jelly coating on the eggs swells due to the absorption of large amounts of water. The swelling of the black jelly causes the eggs to adhere together and protects them from predation by fish and other animals. The fertilized egg, or zygote, undergoes cleavage, forming a tadpole.
