MAMMALS-CHARACTERS

 Mammals (Gr: mammae = mammary glands) are highly evolved tetrapods and they are at the top of the animal kingdom. They were originated in Jurassic period about 180 million years ago from mammal like reptiles, the therapsids (extinctcotylosaurian reptiles).mammals
Various orders of mammals came into existence during the early part of Coenozoic era which is regarded as the Golden age of mammals Mammals are highly successful group of vertebrates. They are found in all parts of the world showing wide range of adaptive radiation.
The class Mammalia includes a variety of animals such as moles, rats, rabbits, bats, cats, whales, horses, camels, elephants, apes, monkeys, man etc. There are about 4000 species of mammals spread over in 27 living orders. Parental care, social behaviour are well developed in the class, Mammalia.
 

GENERAL CHARACTERS OF MAMMALIA

1.Mammals are the warm blooded or homeiothermic (36°C or 98°F) amniotic vertebrates. Inmonotremesthehomeiothermy is imperfect.
2.The body comprises head, neck, trunk and tail. The neck is prominent connecting the head with trunk.
3.The body is usually covered with hair which is epidermal in origin Presence of hair is an important characteristic feature of mammals. Hair is sparcely distributed and secondarily reduced in aquatic mammals like whales. Hair forms a sort of insulation around the body helping in the retention of body heat by radiation.
4.The skin of mammals is thick and water proof. The skin is provided with numerous glands like sweat, sebaceous, scent and mammary glands. Sweat glands (sudorific glands) are epidermal in origin but found in dermis. They regulate the body temperature and also act as excretory organs.
mallals-rabbit
5.Sebaceous glands open into hair follicles and they secrete an oily substance called sebum. The sebum keeps the skin soft and smooth. The sebaceous glands in the pectoral region are modified into mammary glands except in prototherians. The aquatic mammals have a thick layer of fat in the subcutaneous tissue. This fat is known as blubber and serves as a thermal barrier.
 
6.Mammals are tetrapods. Two pairs of pentadactyle limbs, variously adapted for walking, running, climbing, burrowing, swimming and flying, are present. Hind limbs are absent in whales and sea cows. Fore limbs are modified into flippers in aquatic mammals.
7.The head bears a definite nose. Eyes are having movable eye lids with eye lashes to prevent the entry of dust particles into eye. Nictitating membrane is vestigial in mammals (Plica semilunaris). Miebomian, Harderian and Lacrymal glands are present in the eyes.
8.External ears (auditory pinnae) are present. However, they are absent in aquatic mammals, prototherians and ant eaters.
The middle ear contains a chain of bones called ear ossicles. These are malleus (Hammer-like), incus (envil) and stapes (stirrup bone). The internal ear has a long spirally coiled cochlea.
ear-mammal
9.Teeth are firmly fixed in sockets of jaw bones and as such they are called thecodont. All the teeth are not similar and do not carry similar functions. They differ in shape and function. This condition is known as heterodont dentition. There are four kinds of teeth.
a. incisors are always born infront of the jaws and serve for cutting.
b.Canines are long and sharp in carnivorous mammals and help in tearing the flesh. The canines are absent in herbivorous mammals.
c.Premolars are grinding teeth with cusps.
c.Molars (wisdom teeth) resemble premolars in their structure and function. The molars are not represented in milk dentition.
Premolars and molars are together described as cheek teeth.Generally two sets of teeth, the milk set (deciduous set) and a
permanent set are formed in mammals. This condition is
referred to as diphyodont dentition. However, monophyodont
dentition is seen in marsupials.
teeth-mammals
10.The Skull has a large cranial cavity. The number of bones in the skull is reduced and they are firmly united by sutures. The skull is dicondylic. The lower jaw is composed of a single bone called dentary on each side.
11.Mammals possess 7cervical vertebrae with a few exceptions. The centrum is platycoelous or amphiplatyon (centrum is flat in both ends). Intervertebral discs are present between vertebrae. Vertebrae bear epiphyses at both ends of the centrum. The ribs are double headed.
12.Only scapula is well developed in pectoral girdle and the coracoid is very much reduced. An obturator foramen is present in the pelvic girdle.
13.The buccal cavity is divided into an upper nasal passage and a lower food passage by a palate. The respiratory aperture or glottis is guarded by a flap-like epiglottis.
14.There are 4 pairs of salivary glands in the buccal cavity of mammals. In man, there are 3 pairs of salivary glands. The salivary glands are parotid, Sublingual, Sub maxillary and infraorbital. Infraorbitals are absent in Man.
salivary-glands-rabbit
15.The body cavity is divided into an anterior thoracic cavity and a posterior abdominal cavity by a dome-shaped transverse muscular wall called diaphragm.
16. The oesophagus is a long tube. It passes through neck and thorax. It joins the stomach below the diaphragm. There is a caecum at the union of smalj intestine and large intestine. The caecum is long in herbivorous mammals like rabbit and it is short in carnivorous mammals like cat.
17. The liver is five lobed (four lobed in man). The anus is separated from urino-genital aperture. Cloaca is absent in mammals except in monotremes.viscera-rabbit
18. Respiration is affected by a pair of large elastic lungs. A Larynx with vocal cords is usually present. The diaphragm plays an important role in breathing movements.
19- The heart is 4 chambered. Only left aortic (systemic) arch is present. Renal portal system is absent.
mammal-heart
20.The red blood corpuscles are round, biconcave and enucleated. Blood has blood platelets.
21.The brain is of the most advanced type. The surface of the cerebrum possesses convolutions. The two cerebral hemispheres are connected by a transverse nerve band called corpus callosum. There are 4 optic lobes which constitute the corpora quadrigemina
22.The cerebellum bears pons varoli.
23.Twelve pairs of cranial nerves are present.
24.One pair of bean-shaped metanephric kidneys is present in the abdominal cavity on either side of the backbone. A U-shaped Henle's loop is present in the nephron. Nitrogenous waste in mammals is in the form of urea. Hence they are called ureotelic animals.
25.The testes in mammals are situated outside the abdomen in scrotal sacs except in monotremes and whales. The inguinal canal joins the scrotal sac with the abdominal cavity. Males possess a muscular, copulatory organ called penis.
26. Ovaries are situated in the posterior part of abdominal cavity. The oviduct consists of an anterior coiled fallopian tube, a large median uterus and a posterior vagina.
27. A small broad clitoris is situated very close to the urino-genital aperture in female. The clitoris is homologous to the penis of male.
28. Fertilization in mammals takes place in the fallopian tube.
29. Mammals are viviparous. They give birth to young ones (except in Prototheria).
30. The eggs are usually minute without yolk (except in monotremes). Hence, they are called alecithaleggs. The cleavage is holoblastic.
31. . Theembryodevelopsinsidetheuterusofthemotherinallmammals.
The time taken by the young one to develop inside the uterus of the mother is known as the period of gestation' (Intra uterine period). It varies from one mammal to the other.
32. During gestation period the developing embryo establishes an
intimate association with the mother or uterine wall by a structure
called placenta. The placenta serves the function of nutrition,
respiration and excretion.
placenta
33. Foetal membranes like amnion, chorion, allantois and yolk sac
are formed during development. Amnion helps for protection and the others help for respiration, excretion and nutrition.
34. Mammals possess lips and cheeks.
35. Mammals are highly evolved and most intelligent of all animals.

















































No comments:

Post a Comment