Distinguishing Characteristics of Birds

They are homoiothermic i.e. warm blooded animals because they can maintain their body temperature. The body is covered by different coloured feathers which are epidermal exoskeleton. Body is fusiform (streamlined) to allow better movement in air with less resistance. Fore limbs are modified into wings for flight.They have adapted a bipedal life and hind limbs are used for walking on land. The aquatic birds posses webbed feet. Skin is without epidermal glands except for uropygial gland present at the base of tail. Bones of the birds are hollow having air spaces which make them light in weight. Sternum is well developed in to a keel which not only helps in cutting the air during flight but also provide additional area for the attachment of muscles. Jaws are without tecth and are modified in the form of a beak. Digestive system has a crop to store the food and a gizzard to grindit. Heart is four chambered with two auricles and two ventricles. A single aortic arch is present which curves to the right side. Blood is red due to haemoglobin contained in oval, nucleated RBCs. Vocal cords are not present in larynx but a special sound box syrinx is present at the junction of trachea and bronchi. Lugs are provided with extra air sacs. These air sacs are extended in to viscera.Eyes are provided with a third eyelid, the nictitating membrane which can be drawn across the eye. A rudimentary pinna is present outside the external auditory opening. Excretory organs are metanephric kidneys. Ureters open in common cloaca and nitrogenous wastes are excreted in the form of semisolid urates. Sexes are separate and sexual dimorphism is found in many birds. Fertilization is internal. Females has only left ovary and oviduet is well developed. Females have shell secreting glands. Birds are oviparous and have eggs with considerable amount of yolk covered with a hard calcareous shell.. Birds are amniotes and have all the four extra embryonic membranes i.e. amnion, chorion, yolk sac and allontoise. In many birds parental care is very developed.

Types of Birds

Birds are of two types:

a. Flightless Birds

b. Flying Birds

Flightless Birds

are those which do not fly in the air. They are called running birds as

instead of flying they secondarily has adapted a running mode of life. Their bones are not hollow and sternum is not keeled. Tail feathers are irregularly arranged eg.

ostrich, emu, kiwi, cassowary, penguin etc.

Flightless Birds

Flying Birds are the birds with strong wings for flight and keeled sternum. Their bones are hollow. Tail feathers are well developed and are used for steering the bird in

air during flight.e.g. pigeon, sparrow, parrot, eagle, owletc.

Class Mammalia

Mammals are the animals in which females nourish their babies with milk produced by mammary glands. The development of brain and nervous system in mammals is the most remarkable character which has placed them at the top of the evolutionary tree.

Because of possessing solid and hard bones and being the most recently evolved forms, the fossil record of mammals is more continuous and complete. Mammals are believed to be evolved from reptiles. Ancestors of mammals lived simultaneously with reptiles in Jurassic period and are called mammal like reptiles. A fossils animal (named varanope) has been recovered from Texas which has 50% mammalian characters. The ancestors of mammals were of the size of mice and lived on trees. Mammals became dominant in Cenozoic period. Today we are living in the age of mammals.

Following are general characteristics of mammals:

Mammals are warm blooded, air breathing, tetrapods which are mostly terrestrial,

Their body is covered with hairs which insulate the body and help in maintaining

temperature. Mammals have two pairs of pentadactyle limbs which are adapted for

walking, running, climbing, burrowing, swimming, gliding. In aquatic orders hind

limbs are absent. Skin is glandular with sweat glands and sebaceous glands and

mammary glands in females for secreting milk. Brain is well developed with two large

cerebral hemispheres and 12 pairs of cranial nerves. Sense organs are well developed.

Eyes are protected by movable eyelids. Ears have an external pinna for collection of

sound waves. Internal car contains a set of three bones viz. incus, malleus and stapes

which are the smallest bones of the body. Sense of smell, taste and touch are also well

developed.

Respiration takes place through lungs which are spongy in texture due to the presence of air sacs. Larynx is well developed with vocal cords. Heart is four chambered and a complete separation of oxygenated and deoxygenated blood is maintained. Only left aortic arch is present, Colour of the blood is red due to the presence of haemoglobin in biconcave, non nucleated RBCs. Nitrogenous wastes are filtered by highly glomerular kidneys and are excreted in the form of urine. Teeth are present in jaws which have their roots in the jaws. Canine, incisors, premolar and molar type teeth are found in varying numbers in mammals.

Sexes are separate and sexual dimorphism is prominent in most mammals. Testes of male mammals lie in scrotal sac outside the body. Males have a copulatory organ penis for depositing sperms deep inside the vagina of the female. Fertilization is internal and they are viviparous. Embryo is kept inside the body of the female for development, the process is called gestation. To absorb nutrition from the body of mother a glandular tissue develops between foetus and uterine wall of the mother called Placenta. Mammals are also amniotes. The extra embryonic membranes help in the formation of placenta. Parental care is highly developed in mammals.

Mammals are divided into three sub classes.

a. Subclass Prototheria or monotremata

b. Subclass Metatheria or marsupials

C. Subclass Eutheria or placentalia

Subclass Prototheria or monotremata

These are the most primitive mammals and are also called as egg laying mammals. They have certain characters of reptiles like they lay eggs but these eggs are kept in the uterus whre they are fertilized and development takes place. In these mammals there is no connection between the body of mother and foetus for transfer of nutrients.

These animals are more rightly be called as ovo-viviparous. The youngs are given birth in an immature form and are nourished by the teats present on the ventral side of the body in females until they grow enough to survive and start taking their own food. Platypus and spiny ant eater opossum.

Prototheria are restricted to Australian

region and are found in Australia, Tasmania, New Guinea and their neighbouring islands. They are insectivorous, borrowing, nocturnal animal. In adults teeth are absent and a horny beak is found. Their body

temperature varies between 25-28°C.

Example of these mammals are duck

billed platypus (Ornithorhynchus) and

spiny ant eater (Tachyglossus).

b. Subclass Metatheria or Marsupials

These mammals are also called

pouched mammals. Females of these

animals bear a pouch or marsupaim on

the ventral side of the belly in which young ones are kept after hatching as they are

born in a very under developed and immature stage. Teats of the mammary glands are present in the pouch from which the babies suckle milk. In these animals placenta is not formed therefore, babies come out of the body earlier in immature form. Marsupials are also confined to Australian region with the exception of only

one species, American opossum. Their body is covered with hairs. They are terrestrial,

burrowing or arboreal (living in trees) in habit. Examples are Kangaroo, Opossum,

Koala etc.

c. Subclass Eutheria or Placentalia

These animals are the true mammals having the highest degree of evolution of

brain and body structures. They are also called placental mammals as placenta of

different types is formed in these mammals with the help of which the developing

embryo gets nourishment and oxygen from the body of mother and removes its

metabolic wastes. Young ones develop inside the uterus to a relatively mature stage.

After birth mother feeds them on her milk produced in the mammary glands with well

developed teats. Teeth are present in jaws. Cloaca is absent and urino-genital duct

opens independently of rectum. Testes are in scrotum hanging outside the body either

throughout life or at least descend to scrotum during breeding season.

Eutheria are divided into sixteen orders. Some important orders with examples are

mentioned below:

Insectivora:

Feed on insects, includes moles and shrews.

Chiroptera:

Flying mammals like bats, flying squirrels.

Cetacea:

Aquatic mammalse.g. whale, dolphin, porpoises, sea lion etc.

Carnivora:

Flesh eating like dog, cat, lion, Wolves

Rodentia:

Cutting habit like rats, mice, squirrel, beavers etc.

Edentata:

Adults with no or poorly developed molar teeth like South American anteater, sloths.

Body covered with large, overlapping, horney scales e.g. Pengulin

Proboscidea:

Have a long trunk like elephant.

Perissodactyla:

Odd-toed hoofed mammals like, horse, zebra etc.

Artiodactyla:

Even-toed hoofed mammals like cow, goat, deer etc.

Primates.:

Mammals withy highest brain development like lemur, monkeys, apes, tarsiers, human beings etc.

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