AMPHIBIANS

Most people think about amphibians (if they think about them at all) as cold, slimy, and not very bright. However, when amphibians were the most exciting animals on land. They were, in fact, the only animals on land except for some arthropods and an occasional lobe-fined fish.

Representatives of the three major extant groups of amphibians:

Caecilians (order Gymnophiona), such as this yellow-striped caecilian Ichtyophis katouensis, are rarely seen because they are wormlike burrowing animals of the tropics. Caecilians are legless, withe skeletons resembling those of snakes. Like snakes, but unlike other amphibians, some species have dermal scales as protection against abrasion. As in snakes, the lungs are long and thin, and the left one is often much smaller than the right. Some 160 species occur in tropical forests of Central and South America, Africa, India, and the Seychelles islands in the Indian Ocean.

  Ordo that still exists today is:

             Ordo Gymnophiona or Apoda, and other orders are extinct. 

             the example is : 

                                                        picture1. Ichtyophis katouensis    
Salamanders (order Caudata = Urodela) have legs adapted for walking and a tail. Most remain in or close to water, but the adult   Eastern newt Notophthalmus viridescens, known as the red eft, spends several years wandering over land.  Salamanders most resemble the earliest amphibians, with long post-anal tails, and legs of approximatelly equal size suitable for walking. The larvae of urodeles are generally aquatic, with pharyngeal gill slits, external gills, and broad  tails that serve as caudal fins for swimming.
This sub-class consist of 3 ordo, there are:
         Ordo Proteida, the example is:

                             

picture 2a. Necturus lewisi

                                         
                                            picture 2b. Notophthalmus viridescens
                   Ordo Mutabilia, the example is:

                                                 picture  2c. Plethotn tidae



                  
                   Ordo Meanthis, the example is Psandobranchus striatus


  Frogs (order Anura) the most of amphibians. Because of their enlarged hind legs, frogs walk poorly but can hop and swim well. The bullfrog Rana catesbiana is one of the largest and most common frogs of North America. It spends much of its time resting in water with its green head camouflaged among algae and vegetation. The nostrils and upper halves of the eyes are usually above water, while  the lower halves of the eyes watch beneath the surface of the water. Adults frogs range from 1 to 30 cm long and feed mainly on insects and other invertebrates
 

                                                            picture 3a. Rana catesbiana

                                                            picture 3b. Bufo americanus

 The Generall characteristic of amphibians:

• Unlike other lizards, tails of salamanders lack fracture zones. they have the ability to regenerate their limbs.
• Though female toads hardly make noise, male toads are known to be one of the noisiest amphibians.
• Most amphibians are good bio-indicators (organisms that tell something about the area they live in), as they are adapt to conditions in water as well as on land.
• Amphibians possess a three-chambered heart and most species have two lungs (except in some that have no lungs or possess reduced lungs).
• Frogs differ from toads in their habitat and skin texture. Most of their lifetime, frogs live in water and have smooth skin; whereas, toads live mostly on land and possess rough skin.
• A frog in a dormant state requires very little oxygen for breathing, which is achieved through cutaneous gas exchange (breathing via skin).

SKIN

The skin of amphibians sacrifices protection from abrasion and and dehydration to provide a means of exchanging O₂ and CO₂ , especially in an adult frog totally immersed in water. Even on land the skin absorbs almost as much O₂ as the lungs do, and it eliminates most of the CO_2. The respiratory function of the skin depends to a large extent on its thinnes. The epidermis is only a few micrometers thick, permitting easy diffusion of gases to and from the blood capillaries in the underlying dermis. There is an outer layer cornified layer containing the protein keratin, it is generally too thin to provide more than taken protection. 

                                                     picture 4a. skin of frog

Defense. Frog skin is unique in that it is attached to underlying tissues in only a few places. The loose attachment of the skin probably helps frogs escape from predators, especially smaller birds and other animals that cannot swallow a frog whole. Slippery secretion from mucous glands in the dermis probably also helps frogs slip away from predators. The another function is to keep the skin moist so that it remains permeable to respiratory gases. The skin of amphibians also contain poison glands that often concentrated in thickenings of the skin or in bulges behind the eyes called parotoid glands. 

Colors. Many toxic amphibians are brightly colored, and potential predators soon learn to avoid them. Red, orange, yellow warning colors are due to pigmens in certain chromatophores, and blue is a structural color reflected from other kinds of chromotophores. These chromatophores, called melanophores or melanocytes, are also partly responsible for the darkening of the skin which often occurs when amphibians are on a dark substratum or in darkness. Control of the melanocytes is partly due to melanocytes stimulating hormone (MSH) from the pituitary.

                                        

                                                         picture 4b. the poisonous frogs

INTERNAL ORGANIZATION

Lungs. The lungs often account for more than half the oxygen taken into the blood when a frog is on land, and virtually all the oxygen when it is floating in stagnant water with only the eyes and nostrils above the surface. Small amounts of respiratory gases are also exchanged across the lining of the buccal cavity.

                                                       picture 5a. The location of lung

                                                                picture 5b. the lung of frog

Circulation. The amphibians heart has a second atrium that allowa deoxygenated blood from the body tissues to be separated from oxygenated blood coming from the lungs and skin. Deoxygenated blood enters a chamber called the sinus venosus on the back of the heart, which pumps the blood into the right atrium. At the same time oxygenated blood from the skin and lungs enters the left atrium. In some salamanders and frogs, such as Xenopus laevis, the deoxygenated and oxygenated blood mix in the ventricle.

                                                             picture 6. heart of frog

                                                                         

Osmoregulation and Excretion. The kidneys compensate for the loss or gain of water through the skin. The kidneys are the mesonephric type. The urines passes from the kidneys through the ureters into the cloaca and may then be excreted immediately or stored in bladder.  

                                                        picture 7. internal organization of frog