In this article we will discuss about the General Characters and Classifications of Class Mammalia.
General Characteristics of Class Mammalia:
1. These animals are warm blooded, hairy and have mammary or milk producing glands, (mammary glands). They are the only animals which nourish their young ones with milk. There are about 4,000 species of mammals found in the world.
2. They are homoiothermous (warm blooded).
3. Oil glands (sebaceous glands) and sweat glands (sudoriferous glands) are present in the skin.
4. Teeth are of different types (heterodont) and are embedded in the sockets of jaws (the codont). These are developed twice during the life-time of the animal (diphyodont), milk and permanent teeth.
5. Except a few, mammals possess seven cervical (neck) vertebrae.
6. The skull is dicondylic i.e., with two occipital condyles.
7. Respiration is by lungs.
8. They possess a muscular diaphragm dividing trunk into thorax and abdomen.
9. The coelom is divided into four cavities; a pericardial cavity lodging the heart, two pleural cavities each containing the lung and an adominal cavity having the rest of viscera.
10. The heart is four chambered. Sinus venosus is absent. The red blood corpuscles are without nucleus. Renal portal system is absent.
11. The brain has large cerebrum and cerebellum. Optic lobes are divided into four lobes called corpora quadrigemina. Corpus callosum connects the two cerebral hemispheres internally.
12. 12 pairs of cranial nerves are present.
13. Each ear consists of three parts: external, middle and internal. Pinna is a part of external ear. Middle ear has 3 bony ear ossicles (malleus— hammer shaped, incus-anvil shaped and stapes-stirrup shaped). Internal ear has organ of Corti, the actual hearing organ.
14. Except egg laying mammals they are viviparous. There are present four embryonic membranes: chorion, amnion, allantois and yolk sac. Except egg laying mammals a well developed placenta is present.
15. Mammals occur in all sorts of habitats. They are dominant animals and are capable to learn because of their better developed brain.
Oviparous – Omithorhynchus (Duck Billed Platypus), Tachyglossus = Echidna (Spiny Anteater).
Viviparous — Macropus (Kangaroo), Pteropus (Large bat), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Elephas (Elephant), Felis (Cat) Delphinus (Common dolphin), Equus (Horse), Balaenoptera (Blue whale), Panthera tigns (Tiger), Panthera leo (Lion).
Classifications of Class Mammalia:
Living mammals are divided into two sub-classes.
1. Sub-classI. Prototheria:
Prototherians are considered to be the most primitive mammals which are only restricted in Australia and its neighbouring islands (Tasmania New Guinea). Besides egg-laying habit, they have several reptilian characters including a cloaca. They lay eggs containing ample amount of yolk. Subclass prototheria includes one order Monotremata e.g., Omithorhynchus, Tachyglossus- (Echidna).
2. Sub-ClassII. Theria:
They produce young ones. Subclass theria is divided into two infraclasses; Metatheria and Eutheria.
I. Infra-Class Metatheria:
Now they are found mainly in Australia, New Guinea and S. America. Females have a marsupium or brood-pouch for rearing young ones. Infraclass metatheria includes one Order Marsupialia. Mammals of this order are called marsupials or pouched mammals, e.g., Macropus, Didelphis (Opossum) and Phascolarctos (Koala).
II. Infra-class Eutheria:
They are provided with true placenta, hence called placenta! mammals. The embryos are retained in the uterus (womb) till an advanced stage.
Some of the principal orders of placental mammals are briefly described here.
(1) Insectivora (L. insectum- insect, vorare- to eat).
Testes are abdominal. The water shrew is the tiniest mammal which is as large as a human thumb e.g., shrews, moles and hedgehogs.
(2) Dermoptera (Gr. derm- skin, pteron- wing):
A hairy skin fold called patagium extends like a parachute from neck to tail for gliding, e.g., flying lemours. Actually, flying lemurs are neither true lemurs nor do they fly.
(3) Chiroptera (Gk. Cheiros- hand pteron- wing):
They are flying mammals. The forelimbs are modified into wings, e g bats and flying foxes. The vampire bats feed on the blood of mammals including man
(4) Edentata (L edentatus- toothless):
They are toothless. This order includes the armadillos and sloths of South America.
(5) Phoiidota (Gk. pholis- a homy scale):
The body of these mammals is covered with overlapping horny scales with sparse hair in between. Teeth are absent, e.g. Manis (scaly ant eater or pangolin).
(6) Primates (L. primus- of the first rank):
Primates have highly developed brain. The living primates include prosimians (meaning before monkeys) and simians. The prosimians include lemurs, lorises and tarsiers the simians include monkeys, apes and men.
(7) Rodentia (L. rodo- gnaw):
They have one pair of sharp chisel-like incisors in each jaw. The canines are absent, leaving a toothless space, the diastema in the jaw no canines, e.g., rats, mice, squirrels, guinea-pigs and porcupines!
(8) Lagomorpha (Gk. logos- hare, morphe- form):
They have two pairs of incisors in the upper jaw and one pair of incisors in the lower jaw and no canines, e.g., rabbits and hares.
(9) Cetacea (L. cetus- whale):
They have fish-like body, well adapted for aquatic life. They have fin-like fore limbs, but no hind limbs. Testes are abdominal. The skin has a thick layer of fat called blubber serving as reserve food, an insulator for reducing the specific gravity.
Pinnae are reduced or absent. Hair is only on lips. They do not have sweat and oil glands, e g whales, dolphins and porpoises. Blue whale is the largest living animal. Whales normally lack pelvic girdle and hind limbs.
The Green land whales, however, possess vestiges of pelvic girdles and bones of hind limbs inside the body
(10) Carnivora (L. Caro- flesh, vorare- to eat):
They are flesh eating mammals. These animals have sharp pointed canines, strong jaws and well developed claws, e.g., dog, cat, wolf, jackal, fox, cheetah, lion, tiger, hyaena, mongoose, bear, panda, otter, seal, walrus, sea lion. Cheetah is the fastest runner. It can cover a distance of 120 Km in one hour.
(11) Proboscidea (Gk. pro- in front, boskein- to eat):
They have a long muscular trunk. They are thick skinned animals hence called pachyderms (Gk. pachys – thick, derm – Skin). They are the largest land animals, e.g., elephants.
(12) Sirenia (Gk. siren- sea nymph):
They are herbivorous aquatic mammals with fin-like forelimbs and no hind limbs. They have few hairs and do not have external ears.
They have thick blubber. Testes are abdominal. The males have tusks, e.g., Manatee, Seacows.
(13) Perissodactyla (Gk. perissos- odd, dactylos- toes):
They are herbivorous odd-toed hoofed mammals or ungulates (L. ungula- hoof) or hoofed which have an odd number of toes (1 or 3). True horns with a bony core are never present.
The stomach is of nonruminating type (these are not cud chewing animals) e.g., horses, asses, mules, zebras, tapirs and rhinoceros.
(14) Artiodactyla (Gk. artios- even, dactylos- digit):
They are herbivorous even toed hoofed mammals or ungulates (hoofed) which have even number of toes (2 or 4). True horns or antlers are present in many animals of this order. Many even toed hoofed mammals like cow and camel are ruminants or cud-chewing.
The four chambered stomach of cow (Fig. 4.80) is capable of digesting cellulose of plant materials by micro-organisms present in the rumen (first part of their stomach) e.g., cows, buffaloes, sheep, goats, deer, antelopes, yaks, camels, giraffes, pigs and hippopotamuses.