On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate. La grenouille léopard du lac Saint-Pierre Canada s'est avérée contenir des PCB et du Mirex mais en quantité à ce jour considérée comme non dangereuse pour les consommateurs. This can be represented by the following chemical equation:. These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed.
What is a Mammal?
Other skin secretions, especially toxins , have potential use as anesthetics and painkillers. Biochemists are currently investigating these substances for medicinal use. The three living groups of amphibians have distinct evolutionary lineages and exhibit a diverse range of life histories. The breeding behaviour of each group is outlined below.
One similar tendency among amphibians has been the evolution of direct development, in which the aquatic egg and free-swimming larval stages are eliminated. Development occurs fully within the egg capsule, and juveniles hatch as miniatures of the adult body form.
Most species of lungless salamanders family Plethodontidae , the largest salamander family, some caecilians, and many species of anurans have direct development. In addition, numerous caecilians and a few species of anurans and salamanders give birth to live young viviparity. Anurans display a wide variety of life histories. Centrolenids and phyllomedusine hylids deposit eggs on vegetation above streams or ponds; upon hatching, the tadpoles anuran larvae drop into the water where they continue to develop throughout their larval stage.
Some species from the families Leptodactylidae and Rhacophoridae create foam nests for their eggs in aquatic, terrestrial, or arboreal habitats ; after hatching, tadpoles of these families usually develop in water.
Dendrobatids and other anurans deposit their eggs on land and transport them to water. Female hylid marsupial frogs are so called because they carry their eggs in a pouch on their backs. A few species lack a pouch and the tadpoles are exposed on the back; in some species, the female deposits her tadpoles in a pond as soon as they emerge. Inside the egg, the embryo is enclosed in a series of semipermeable gelatinous capsules and suspended in perivitelline fluid, a fluid that also surrounds the yolk.
The hatching larvae dissolve these capsules with enzymes secreted from glands on the tips of their snouts. The original yolk mass of the egg provides all nutrients necessary for development; however, various developmental stages utilize different nutrients. In early development, fats are the major energy source. During gastrulation , an early developmental stage in which the embryo consists of two cell layers, there is an increasing reliance on carbohydrates.
After gastrulation, a return to fat utilization occurs. During the later developmental stages, when morphological structures form, proteins are the primary energy source. By the neurula stage, an embryonic stage in which nervous tissue develops, cilia appear on the embryo, and the graceful movement of these hairlike structures rotates the embryo within the perivitelline fluid. The larvae of direct developing and live-bearing caecilians, salamanders, and some anurans have external gills that press against the inner wall of the egg capsule, which permits an exchange of gases oxygen and carbon dioxide with the outside air or with maternal tissues.
During development, ammonia is the principal form of nitrogenous waste, and it is diluted by a constant diffusion of water in the perivitelline fluid. The development of limbs in the embryos of aquatic salamanders begins in the head region and proceeds in a wave down the body, and digits appear sequentially on both sets of limbs. Salamanders that deposit their eggs in streams produce embryos that develop both sets of limbs before they hatch, but salamanders that deposit their eggs in still water have embryos that develop only forelimbs before hatching.
In contrast, the limbs of anurans do not appear until after hatching. Soon after the appearance of forelimbs, most pond-dwelling salamanders develop an ectodermal projection known as a balancer on each side of the head.
These rodlike structures arise from the mandibular arch, contain nerves and capillaries, and produce a sticky secretion. They keep newly hatched larvae from sinking into the sediment and aid the salamander in maintaining its balance before its forelimbs develop.
After the forelimbs appear, the balancers degenerate. During the embryonic and early larval stages in anurans, paired adhesive organs arise from the hyoid arch, located at the base of the tongue.
The sticky mucus they secrete can form a threadlike attachment between a newly hatched tadpole and the egg capsule or vegetation. Consequently, the tadpole that is still developing can remain in a stable position until it is capable of swimming and feeding on its own, after which the adhesive organs degenerate.
The amphibian larva represents a morphologically distinct stage between the embryo and adult. The larva is a free-living embryo. It must find food, avoid predators, and participate in all other aspects of free-living existence while it completes its embryonic development and growth. Salamander and caecilian larvae are carnivorous , and they have a morphology more like their respective adult forms than do anuran larvae. Not long after emerging from their egg capsules, larval salamanders, which have four fully developed limbs, start to feed on small aquatic invertebrates.
The salamander larvae are smaller versions of adults, although they differ from their adult counterparts by the presence of external gills, a tailfin, distinctive larval dentition, a rudimentary tongue, and the absence of eyelids. Larval caecilians, also smaller models of adults, have external gills, a lateral-line system a group of epidermal sense organs located over the head and along the side of the body , and a thin skin.
In anurans, tadpoles are fishlike when they hatch. They have short, generally ovoid bodies and long, laterally compressed tails that are composed of a central axis of musculature with dorsal and ventral fins.
The mouth is located terminally recessed , ringed with an oral disk that is often fringed by papillae and bears many rows of denticles made of keratin. Tadpoles often have horny beaks. Their gills are internal and covered by an operculum. Water taken in through the mouth passes over the gills and is expelled through one or more spiracular openings on the side of an opercular chamber.
Anuran larvae are microphagous and thus feed largely on bacteria and algae that coat aquatic plants and debris. Salamander larvae usually reach full size within two to four months, although they may remain larvae for two to three years before metamorphosis occurs. Some large aquatic species, such as the hellbender Cryptobranchus alleganiensis and the mud puppy Necturus maculosus , never fully metamorphose and retain larval characteristics as adults see below heterochrony.
Tadpole development varies in length between species. Some anuran species living in xeric dry habitats, in which ephemeral ponds may exist for only a few weeks, develop and metamorphose within two to three weeks; however, most species require at least two months. Species living in cold mountain streams or lakes often require much more time.
For example, the development of the tailed frog Ascaphus truei takes three years to complete. These changes mark the transformation from embryo to juvenile and the completion of development. Hormones ultimately control all events of larval growth and metamorphosis, and in many instances, development is accompanied by a shift from a fully aquatic life to a semiaquatic or fully terrestrial one.
Although salamanders undergo many structural modifications, these changes are not dramatic. The skin thickens as dermal glands develop and the caudal fin is resorbed. Gills are resorbed and gill slits close as lungs develop and branchial gill circulation is modified. Eyelids, tongue, and a maxillary bone are formed, and teeth develop on the maxillary and parasphenoid bones. Changes that occur in caecilians—the closure of the gill slit, the degeneration of the caudal fin, and the development of a tentacle and skin glands—are also minor.
Skeletal changes are much more dramatic in anurans because tadpoles make an abrupt and radical transition to their adult form. Limbs complete their development, and the forelimbs break through the opercular wall, early in metamorphosis. The tail shrinks as it is resorbed by the body, dermal glands develop, and the skin becomes thicker. As lungs and pulmonary ventilation develop, gills and their associated blood circulation disappear.
Adult mouthparts replace their degenerating larval equivalents, and hyolaryngeal structures develop. The shed tail can be regenerated within a few weeks, but it is smaller than the original one and looks quite different from the rest of the body. The principal defense mechanism in various snake species is their ability to deliver poison through their fangs.
Poison glands located inside their mouth produce deadly venom that helps the creatures to protect themselves from enemies.
These animals originated around million to million years ago with the first reptiles evolving from the advanced reptiliomorph labyrinthodonts. Animals from the Casineria genus are the earliest animals suspected to be amniotes rather than advanced amphibians.
The origin of reptiles occurred in steaming swamplands of late Carboniferous period. Fossils found in Nova Scotia show footprints with imprints of scales and typical reptilian toes. The prints have been attributed to Hylonomus which is the earliest recognized reptile. Hylonomus was a lizard-like creature 8 to 12 in long with sharp teeth that indicates an insectivorous diet.
Earlier, the bigger labyrinthodont amphibians like Cochleosaurus largely overshadowed the reptiles, who remained inconspicuous until the CRC or Carboniferous Rainforest Collapses. This significant extinction event affected the existence of many large animals with the Amphibians being among the worst affected populations. But, the reptiles coped better with the drier conditions post CRC. One of the biggest problems the Amphibians faced was the lack of water bodies which prevented them from reproducing.
This did not affect reptiles as they were able to lay eggs on dry lands. Gradually, the reptiles became a dominating class with new diet strategies including both carnivory and herbivory. This established the foundation for the Mesozoic stage, also referred to as the Age of Reptiles. The genus Mesosaurus from the Permian period is counted among the most recognized early reptiles.
Most Reptiles are unable to see properly during nighttime as their vision is mainly adapted to the daylight conditions. They have color vision with the visual depth perception being much more advanced than Amphibians and many Mammals. The vision is reduced in species like the Blind Snake, while some snakes have extra visual or sensory organs that make them sensitive to heat and infrared radiation.
The horny epidermis layer makes their skin watertight, allowing these animals to inhabit dry land. Reptiles have thinner skin compared to mammals and it also lacks the dermal layer present in mammal skin. The exposed skin areas are covered in scutes or scales which may have a bony base, creating their armors.
In turtles, a hard shell made up of fused scutes covers the entire body. Reptiles use their lungs for breathing.
The skin of the aquatic turtles is more permeable for allowing them to respire while the cloaca is modified in various species to increase the gas exchange area. Despite these adaptations, lungs remain a very important part of their respiratory system.
The main Reptile groups accomplish lung ventilation in different procedures. Squamates are known to ventilate the lugs mainly by their axial musculature.
Certain lizard species are capable of buccal pumping apart from the normal axial breathing. The proto-diaphragm in Tegu lizards separates their pulmonary cavity from visceral cavity, helping with their respiration by allowing greater lung inflation.
The muscular structure of the diaphragm in the Crocodilians species resembles that of various mammals. However, there are some differences in their diaphragmatic setup.
They also have two aortas playing a major role in their systemic circulation. The oxygenated and deoxygenated blood may get mixed with each other in their three-chambered heart with the level of mixing depending on the species and the physiological state of the animal.
Their circulatory system is capable of shunting back the deoxygenated blood to the body and the oxygenated blood to the lungs if necessary. Unlike other Reptiles, animals in the crocodilian subgroup have four-chambered hearts. But, their two systemic aortas are only capable of bypassing their pulmonary circulation. On the other hand, the three-chambered hearts in various lizard and snake species can function as the four-chambered ones during contraction. Majority of these animals have short digestive tracts because their diet mainly consists of meat, which is very simple to digest.
Their digestion process is slower than that in mammals due to their inability of mastication and their low metabolism rate while resting. The energy requirements for their poikilotherm metabolism are very low which allows large animals from this class such as various constrictors and crocodiles to survive for months from one large meal, digesting it slowly. Herbivorous reptiles are also unable to masticate their food, which slows down the digestive process.
Some species are known to swallow pebbles and rocks that help in grinding up plant matters within the stomach, assisting their digestion. The basic nervous system in the Reptiles is similar to that in the Amphibians. But, Reptiles have slightly larger cerebrum and cerebellum.
Most of the important sensory organs are properly developed in these creatures. However, there are certain exceptions such as the absence of external ears in snakes they have the inner and middle ears.
Reptiles have twelve cranial nerve pairs. They have to use electrical tuning for expanding the range of their audible frequencies because they have short cochlea. These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter. However, the brain development can be more complex in some larger Reptiles. Modern species also have pineal glands in their brains. Most of these animals are tetrapods, meaning they have four legs.
Snakes are examples of legless Reptiles. Their skeletal system is similar to other tetrapods with a spinal column supporting their bodies. Their excretory system consists of two small kidneys. The diapsid species excrete uric acid as the principal nitrogenous waste product. But, turtles excrete mainly urea. Some of these species use their colons for reabsorbing water, while some are able to absorb the water stored in their bladders.
Certain Reptiles excrete the excess salts in their bodies through the lingual and nasal salt glands. Reptiles have certain characteristic features that help in distinguishing them from Amphibians, Mammals and Aves:. However, these gland and soft tissue features cannot be used for identifying fossils of prehistoric mammals. The following anatomical traits are observed for this purpose:.
Mammals are found in all types of habitats and they can adapt to almost all climatic conditions. They inhabit tropical rainforests, savannas, temperate regions and extremely cold climates. Many species live in underground burrows while others dwell on trees. Majority of the species live on land and can be found in forests, deserts, grasslands and mountains.
Most of the well known Mammals, like lion, tiger, bear and dog are land animals. There are around different species including whales, seals, walruses and dolphins that live in ocean habitats. But they do not inhabit very deep waters as they need to resurface from the water regularly to get oxygen.
Many species are arboreal, meaning they live mainly in trees. Squirrel, sloth, marmoset and koala are examples of arboreal Mammals.
Most of these species are viviparous and give birth to live offspring. The five monotreme species including the platypuses and echidnas are the only egg-laying Mammals. Almost all the species practice sexual reproduction. The sex determination system of the monotremes is quite different than that of many other animals from this class. Viviparous Mammals are included in the Theria subclass with only two extant infraclasses; the marsupials and the placentals.
The gestation period is very short in the marsupials such as kangaroo , with the animals giving birth to undeveloped newborns. The placental species generally have long gestations periods and give birth fully developed young. Elephants have the longest gestation period among Mammals, which lasts for around 22 months. The number of offspring born can vary from one to many depending on the species. They do not undergo any larval stage or metamorphosis to become adult animals.
Mammal young are born as miniature versions of their adults with little physical differences. The babies are taken care of by the adults until they become capable of fending for themselves.
The underdeveloped marsupial babies usually undergo further development within the marsupium the pouch-like sac located at the front side of the female's abdomen. The physical growth of the young Mammals continues until they reach the adult age. The adult females have mammary glands that produce milk, which is fed to the young as their primary nutrition source.
The babies of the egg-laying Mammals lick the milk secreted by the mammary glands of the females onto the mammary patch located on their bellies. Animals belonging to this class have different adaptive features to survive in their respective surroundings and climatic conditions.
Mammals belonging to the Order Chiroptera bats exhibit special adaptive features that enable them of true flight:. Many species such as the Crescent nail-tail wallaby, Steller's sea cow and Flores cave rat have even faced complete extinction. The overall Mammal population throughout the world is still deteriorating with around one-quarter of the total species being threatened with extinction.
The wild life conservation units in different countries are taking important measures for protecting different species from the class Mammalia. Various other species are also protected by law in different countries. The Chinese Water Deer also known as the Asian water deer is a small ungulate that is known for its long fang-like canine teeth and belongs to the w.
The Pygmy Hippopotamus is a species of dwarf or very small hippopotamid, compared to the other common hippo species. By size, these mammals are about. The Bat-eared Fox is a species of very ancient canines that was widely distributed in the middle Pleistocene era, around , years ago. The Himalayan Tahr, also known as kaarth, meshi, and taheer, is a species of grazing ungulates that are found in parts of the Himalayan mountain range.
The Indian wolf is a grey wolf subspecies found in west and South Asia. It was initially given species status as Canis pallipes but was assigned as a. Javan rhinoceroses are rhinos that had a historical range across Java, Sumatra, Southeast Asia, all the way through China into India.
Sumatran tigers are a tiger subspecies found in the Indonesian island of Sumatra. They are the smallest of all tigers, being comparable in size to a l. The African Bush Elephant is the largest and heaviest of the land mammals in the world. They are scattered roughly throughout the savanna region of th.
The Hooded Seal, also called the Bladder-nosed Seal, is a species of large silver-grey seal found in the cold waters of the Atlantic. The seal gets it. Reproduction in whole or in part without permission is prohibited. Mammal Classification At present, there are more than 5, extant species belonging to the Mammalian class. Following is the general classification of the animals: Family Ornithorhynchidae the duck-billed platypus Family Tachyglossidae the spiny anteaters. Evolution of Mammals Amniotes were the first vertebrates to be fully terrestrial.
Anatomy of Mammals Skeletal System Majority of these animals, including giraffes, bats, whales and humans, have 7 cervical vertebrae neck bones. Respiratory system Animals belonging to this class respire by inhaling oxygen and exhaling carbon dioxide.
Nervous system The brains of the placental mammals possess a corpus callosum, which is absent in monotremes and marsupials. Skin The Integumentary system or skin is comprised of three layers: