Chordata-Amphibia
First off, all amphibians are cold-blooded and lay eggs, rather than live offspring. They also all go through metamorphosis, meaning they aren't born like we see them in the picture presented on the left. This is actually where this class gets its name from, amphibian means "two lives". Almost all of our amphibianas live in the tropical areas, meaning they live in warmer environments. They are however found all over the world, even in Antarctica and Greenland (although they are sparce in these areas).
Amphibians are thought to have evolved around 400 millions years ago over a very long period of about 50 million years. They evolved originally from fish, that developed lungs, making it to where they could live in shallow waters. After millions of years after that they made their way to walking on land as well, and millions of years after that, became the worlds first amphibians.
All amphibians have a three-chambered heart (different from mammals who have four chambers), and breathe through their lungs or skin. The three chambers of the heart are two atria and one ventricle. The circulatory system has two circuits that distribute blood throughout the body to organs under high pressure, called the pulmocutaneous circuit and the systemic circuit. Most amphibains have four legs, with the exception of the apoda who do not have any legs at all. Amphibains can live on either land or water, mostly because of their offspring, which must be layed in the water to survive.
The class of Amphibians is a large one consisting of many different kinds of species taking many different shapes. Urodela ("tailed ones") consist of all the salamanders. There are around 500 different species of salamanders around today. Mostly, salamanders are found in the northern hemosphere, living in continents such as North America (especially the Great Smokey Moutains area), Europe and Asia. Anura ("tail-less ones") is the category in which frogs live under. There are around 4,200 different species of frogs because frogs are better at adapting. to land than salamanders are. Frogs can live from deserts to rainforests and can even be poisoness, a great weopon against predators. Apdoa ("legless ones") and are very few in species, numbering only about 150 species total. These creatures resemble a mix or worms and snakes and live underground for the most part. Below is a picture of a group of caecilians.
For aquiring and digesting food, the baby stages for frogs are herbivores, however by the time they have gone through metamorphosis, they have a carnivorous diet. Mostly, amphibians like to eat insects, however they have been known to eat about anything that they can swallow, including rodents, fish, snakes, birds and even canabolistic, and eat other frogs and salamanders. Caecilians are different, as they eat earthworms and other organisms of the such found other the ground. Almost all amphibians are noctornal, meaning that they hunt at night. Amphibians do not actually have very many predatory characteristics, including claws, talons, etc. So, to catch their prey ( frogs and salamanders) catch their prey with their tongues. They attack the prey with thier tongue, bring the prey towards them, and swallow them whole. Frogs however do have teeth on the upper portion of their jaw, that they do use before swalling thier food. Amphibians rid themselves of waste by excretion of diluted urine from their kidneys whenever salts collect on the skin of the amphibian. Due to the fact that it is imperitive for land amphibians to be hydrated at all times, frogs reabsorb the water on the epithelium tissue of the urinary bladder in order to retain body fluid.
Amphibians are extremly aware of their environment, they have to be for their survival. Frogs and salamanders are able to see shapes and colors, however both are limited. Caecilians live underground, so thier eyesight has adapted to the dark environment. Also, caecilians contain a tentacle between their eyes and nose to help with their sense of smell, because they have limited sunlight. Amphibians do not contain ears that we can see, but instead have ears within thier boides, allowing them to hear in their environment.
Salamanders use their tails and their four legs to move around, however their "walking" is different then one would imagine. The Urodela move around with each side walking as a unit, so the entire left side of the body moves forward, then the right side. Tadpoles, or the young stages of a frog's life uses their tails to move around, however the adults use thier hind legs. I would like to make a note here, toads and frogs are all actually frogs and belong in the same family (Bufonidae). Continuing, frogs use thier hind legs to hop. Apoda's move around like snakes do, sliding across the ground.
Dehydration is very dangerous for amphibians, as they need this water to survive, however they are constantly in danger of drying out because of their smooth skin. To stay moisturized, amphibians stay in moist areas, and are aware when they are drying up. If an amphibian notices that they are dehydrated then they will move to areas of shade and water. Some amphibians, mostly frogs, are able to produce a layer of mucus over their skin to protect themselves from the heat. If amphibians are in the water, they obtain water through their skin, thanks to osmosis. Amphibians, for the most part, do not actually need to drink water, but more soak in the necessary water through their skin.
Amphivians are able to protect themselves from their numerous predators in a number of ways. One method that expecially frogs like to use, is secreting a deadly toxin from thier skin if threatened. Other predators know if a frog is poisoness like that because of their bright colors. Other frogs pretend to be poisoness with their bright colors, even though they are unable to produce any toxins. Some salamanders are known to lose thier tales as a way to confuse a predator. Other ways amphibians protect themselves is by blending in with it's environement, so that a predator cannot see them.
When amphibians reproduce, they lay eggs. Amphibian eggs do not have a hard shell like a bird's eggs, but rather they have gelatinous layers. They are fertilized outside of the female's body; a female will lay her eggs in water and the male sprays sperm over the eggs. Where an anphibian keeps fertilized eggs differs between species, but they can be kept in the mouth or stomach or on the back of either the male or female. Depending of the species of amphibian, the eggs can be laid in large numbers all at once with a higher chance of the mother dying or a few at a time with the parents staying with the eggs for a period of time, the length of that time also depending on the species.
Works Cited
https://sharon-taxonomy2009-p3.wikispaces.com/Chordata-Amphibia
