EUTHERIA – METATHERIA-PROTOTHERIA-CHARACTERS

DISTINCTIVE FEATURES OF PROTOTHERIA

1. The prototheria are described as primitive unfinished mammals by Romer.

2. The mammary glands of prototheria are modified swear glands unlike the Eutheria. Teatless mammary glands are characteristic of prototheria.

3. Males possess functional mammary glands. The feeding of young with milk by father and mother (parents) 'is known as gynaecomastism.

prototheria-duckbill-ornithorhynchus

4. Auditory pinna or external ear is absent.

5. The skull is without tympanic bulla and lacrimals. Vertebrae are without epiphyses.

6. Cervical vertebrae bear ribs. Ribs are single headed having capitulum only. A large coracoid with precoracoid and a T shaped interclavicle is present in pectoral girdle.

7. The pelvic girdle has a pair of epipubic bones which are absent in eutherians.

8. The body temperature is low and ranges from 25° C to 28°C. Hence, they are described as heterothermous mammals.

9. A cloaca is present into which ureters and urinogenital ducts open. Oviducts remain separate throughout the length and open into the cloaca.

10. Testes are abdominal. Penis helps to passout the sperms only.

11. There is no corpus callosum in the brain.

12. Teeth are present only in young ones; adults lack teeth, but a horny beak is present.

13. Females are oviparous laying large eggs with much amount of yolk (megalecithal).

14. Optic lobes are 4 in number.

15. Prototherians possess a number of reptilian characters in addition to the mammalian characters. So they form the bridge to fill the gap between reptiles and mammals, i.e they act as connecting link between Reptiles and Mammals.

Reptilian features of Monotremes

1. A large coracoid with precoracoid and T-shaped interclavicle in pectoral girdle.

2. Pelvic girdle is with epipubic bone.

3. Vertebrae are without epiphyses. Ribs are single headed.

4. Cloaca is present and testes are abdominal.

5. Laying of large eggs with abundant yolk.

6. Meroblastic cleavage is seen in the development.

Mammalian features of Monotremes

1. Hair is present all over the body.

2. Diaphragm is present.

3. Only left aortic arch is present.

4. Heart is 4 chambered.

5. Non-nucleated red blood corpuscles are seen in blood.

6. Mid brain shows four optic lobes.

7. Skin possesses mammary, sweat and sebaceous glands.

8. Three ear ossicles are present in the middle ear.

9. A single bone (dentary) is present in each half of the lower jaw.

10. A slightly coiled cochlea is present.

This sub-class Prototheria includes only one living order Monotremata. It includes 3 living genera. These are found in Australia, Tasmania and New Guinea.

1. Ornithorynchus or duck billed platypus: It is distributed in Australia and Tasmania.

2.Echidna or Tachyglossus or spiny ant eater:

It is found in Tasmania and New Guinea.

3.Zaglossus or Proechidna : It is found in New Guinea and resembles Tachyglossus.

DISTINCTIVE FEATURES OF METATHERIA- MARSUPIALS

Marsupials are more advanced than the prototherians. They are all pouched mammals characterised by the presence of an integumentary brood pouch or marsupium in which the immature young ones are fed with the milk of the mother.

The marsupials occupy an intermediate position between the primitive mammals (monotremes) and the higher mammals (eutherians).

1. Marsupium or brood pouch is present in females. The mammary glands are present in the marsupium.

2. The marsupium is supported by epipubic (marsupial) bones.

3. The teeth are more in number. There are more than 3 incisors and more than 4 molars in each half of the jaw. Teeth are formed only once in life time. Hence, the dentition is called monophyodont dentition.

marsupialia-kangourou

4. Corpus callosum is usually absent in the brain and Placenta is absent.

Although the anal and urinogenital apertures are separate and distinct, they are surrounded and controlled by a common sphincter muscle.

5. Testes are situated in scrotal sacs. The penis is present behind the scrotum and it is bifid.

6. The females have 2 oviducts, 2 uteri and 2 vaginae (didelphic condition) which separately open into urinogenital sinus.

7. The young ones are born naked and blind, but they posses clawed fore limbs by which they move into the brood pouch.

8. Fertilization and a major part of development are internal, but the young ones are born in immature state (mammary foetus).

10. Yolk sac placenta is present in marsupialia. True allantoic placenta is absent (except parameles). Yolk sac is large with villi.

11. Hind limbs are long in some animals like kangaroo.

12. The body temperature ranges from 36° C to 40° C.

13. Marsupials exhibit discontinuous distribution. Some marsupials like kangaroos live in Australia and oppossums live in South America. Australia is described as the land of marsupials.

Eg : Thylacinus (Tasmanean wolf) is a carnivorous marsupial.

Notoryctus (Marsupial mole) is a burrowing form. Phascolarctus (koala bear) is an arboreal form. Myrmecobius (banded ant eater). Parameles is a marsupial bandicoot. Macropus (Kangaroo) is the largest marsupial. | Didelphls marsupialis (American opposum).

EUTHERIA OR PLACENTALIA- SALIENT FETURES

1. These are highly evolved mammals with advanced organisation.

2. Marsupium and marsupial bones are entirely absent.

3. Nourishment, respiration and excretion of young takes place through a complex allantoic placenta.

4. The young ones are born in relatively advanced state (miniature adults). Complete development takes place in the uterus only. Gestation period is generally long.

5. Testes are usually present in scrotal sacs. Penis is situated infront of the testes.

6. Vagina is single. Urinogenital passage and anus are separate.

EUTHERIA

7. Cerebral hemispheres in brain have more convolutions.

8. Corpus callosum is well developed.

9. Eutherians are viviparous.

10. Its distribution is world-wide.

Eg : Rats, bats, rabbits, monkeys, cats, dogs, whales, horses, deers, pigs, elephants, man etc.

3 comments:

  1. Thanks for the organised answers.it is indeed helpful.

    ReplyDelete
  2. Thanks for the organised answers.it is indeed helpful.

    ReplyDelete