The thin skin of amphibians contains many glands, among them poison glands that protect certain species against predators. The poison from the glands of the brightly colored poison-dart frog is particularly toxic and is used by South American Indians to coat the tips of their arrows. Some amphibians protect themselves from enemies by changing color to blend in with their surroundings.

Characteristics:it can breathe underwater using its moist skin

Birds are vertebrates, which means that they are among those animals that possess a backbone. They range in size from the minute Cuban Bee Hummingbird (Calypte helena) to the grand Ostrich (Struthio camelus). Birds are endothermic and on average, maintain body temperatures in the range of 40°C-44°C (104°F-111°F), though this varies among species and depends on the activity level of the individual bird.

Birds are the only group of animals to possess feathers. Feathers are used in flight but also provide birds with other benefits such as temperature regulation and coloration (for display and camouflage purposes). Feathers are made of a protein called keratin, a protein that is also found in mammalian hair and reptilian scales.

The digestive system in birds is simple but efficient (enabling them to pass food through their system quickly to minimize the extra weight of undigested food and the time it takes to extract energy from their food). Food travels through the parts of a bird's digestive system in the following order before it is excreted!

What about mammals that don't have any visible body hair, like whales or Olympic swimmers? In the case of ​whales and dolphins, many species have sparse amounts of hair during the earliest stages of their development, while others retain wispy patches of hair on their chins or upper lips. And, of course, even completely hairless-looking humans still retain the hair follicles in their skin!

Unlike other vertebrates, mammals nurse their young with milk produced by mammary glands. Though they're present in both males and females, in most mammal species mammary glands only fully develop in the females, hence the presence of smaller nipples on males (including male humans). The exception to this rule is the male Dayak fruit bat, which nature has endowed (for better or worse) with the task of breastfeeding.

Mammary glands are modified and enlarged sweat glands consisting of ducts and glandular tissues that secrete milk through nipples; the milk provides young with much-needed proteins, sugars, fats, vitamins, and salts. However, not all mammals have nipples: monotremes like the platypus, which diverged from other mammals early in evolutionary history, instead secrete the milk produced by their mammary glands via ducts located in their abdomens.
The lower jawbone of mammals is composed of a single piece that attaches directly to the skull. This bone is called the dentary, because it holds the teeth of the lower jaw.; in other vertebrates, the dentary is only one of several bones in the lower jaw, and does not attach directly to the skull. So what's the big deal? Well, this single-pieced lower jaw and the muscles controlling it endows mammals with a powerful bite, and also allows them to use their teeth to either cut and chew their prey (like wolves and lions), or grind down tough vegetable matter (like elephants and gazelles).

The first and foremost characteristic of the reptiles is that are cold blooded. They are able to regulate their inner body temperature to the temperature of the environment. They have scaly skins but absence of hair or fur. The scales of the reptiles develop as a surface cells filled with Keratin.

Most of the reptiles have backbone or spinal columns except snakes. All the reptiles have pelvis that is associated with at least two spinal bones.

Reptiles lay eggs to give birth to the young ones but of the reptiles are capable of giving birth to the young ones (i.e. internal fertilization). Reptiles lay amniote eggs that are capable of living outside the water. These amniote eggs have complex membrane system and protective shells that are porous to oxygen and other gases. The young ones of reptiles directly hatch into young adult without going to the larval stage or experience metabolism as in amphibians.

Reptiles have three chambered heart except alligators and crocodiles which have four chambered heart. They have two aortic blood vessels carrying blood from the heart to all other parts of the body. Well developed lungs that is folded into small sacs called alveoli helps the reptiles to breathe. All the reptiles breathe through their lungs throughout their lifetime. These alveoli greatly increase the internal surface area and increase the effectiveness of the lungs.

Reptiles also posses metanephric kidneys, twelve pairs of cranial nerves, at least two spinal bones, one ball and socket connection.

Most of the reptiles are tetrapods (four legged animals). They have claws on their toes. Reptiles such as snakes and some lizards only are legless.

Regarding the sense organs the reptiles rely on their vision than any other sense organs except snakes which are blind. The reptiles use their vision to detect objects around their surroundings. All the reptiles lack external ears instead they have eardrum that is positioned neat the eyes and is placed close to the surface of the skin.

Fish come in a wide variety of colors, shapes, and sizes - there's the largest fish, the 60+ foot long whale shark, popular seafood fish such as cod and tuna, and completely different-looking animals such as seahorses, sea dragons, and pipefish. In all, about 20,000 species of marine fish have been identified.


Fish swim by flexing their bodies, forming waves of contractions along their muscles. These waves push water backward and move the fish forward.

One of the most notable features of fish are their fins - many fish have a dorsal fin and anal fin (near the tail, on the underside of the fish) that provide stability. They may have one, two or even three dorsal fins. They may also have pectoral and pelvic (ventral) fins to help with propulsion and steering. They also have a caudal fin or tail.

Most fish have scales covered with a slimy mucus that helps protect them. They have three main types of scales: Cycloid (roundish, thin and flat), ctenoid (scales that have tiny teeth on their edges), and ganoid (thick scales that are rhomboid in shape).

Fish have gills for breathing - the fish inhales water through its mouth, which passes over the gills, where hemoglobin in the fish's blood absorbs oxygen.

Fish may also have a lateral line system, which detects movement in the water, and a swim bladder, which the fish uses for buoyancy. 

The fishes are divided into two superclasses: Gnathostomata, or vertebrates with jaws, and the Agnatha, or jawless fishes.

With thousands of species, reproduction in fish can be remarkably different. There's the seahorse - the only species in which the male gives birth. And then there are species like cod, in which females release 3-9 million eggs into the water column. And then there are sharks. Some shark species are oviparous, meaning they lay eggs. Others are viviparous and give birth to live young. Within these live-bearing species, some have placenta-like human babies and others do not.