Class mammalia is divided into 2 subclasses: Prototheria and Theria. The subclass Prototheria includes primitive egg-laying mammals belonging to a single order, Monotremata.
Monotremes (from the Greek monos “single” + trema “hole”, referring to the cloaca) are mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). The only surviving examples of monotremes are all indigenous to Australia and New Guinea, although there is evidence that they were once more widespread. Among living mammals they include the platypus and the echidnas (or spiny anteaters).
The distribution of Monotremes is restricted. They are found in Australia, Tasmania and New Guinea.
- Body is covered with soft spine-like hairs.
- Muzzle or snout produced into a beak.
- External ear not noticeable.
- Digits end in sharp claws and are webbed.
- Mammary glands are devoid of teats. The tubular glands open in shallow depression on the ventral side.
- Tail may be present or absent.
- In males, poison spur is present.
- The brain lacks corpus collosum (It is a wide, flat bundle of neural fibers, It connects the left and right cerebral hemispheres and facilitates interhemispheric communication).
- The eggs are large and undergo meroblastic segmentation.
- Skull dicondylic.
- The sutures of skull are obliterated.
- Auditory ring is incomplete.
- Ear ossicles three.
- Pterygoid bone is present
- The mandible is slender.
- The scapular spine is placed at the anterior border.
- Angular and coracoid processes are ill-developed.
- Mandibular symphysis is absent.
- Teeth are replaced by a horny pad in adults. Young possess teeth. Dental formula in young stage is 0.1.2.3./22.214.171.124.
- The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different
1. Epiphysis is ill-developed.
2. Zygapophyses in cervical vertebrae are ill-developed.
3. Thoraco-lumbar vertrbrae are 19 and sacral 2-4 in number.
4. Cervical ribs are present.
- It is built in typical reptilian plan.
- Scapula is elongated and is without spine.
- Coracoid is large.
- The monotremes also have extra bones in the shoulder girdle, including an interclavicle and coracoid, which are not found in other mammals
- Acetabulum is perforated.
- Epipubic bone is present.
- Humerus is flat having a little developed olecranon process.
- Femur is flat with prominent trochanter.
- Patella is large.
- Tongue is long and sticky.
- Sweat and sebaceous glands are present.
- Testes abdominal. Erectile penis is present in males.
- Right ovary reduced in females and paired oviducts open separately into cloaca. No uterus and vagina present.
- Females oviparous. Leathery shelled eggs contain much yolk.
- Heart four chambered. Right auriculoventricular valve incomplete and fleshy. Right valve is not bicuspid like other mammals. It is tricuspid.
- Metanephric kidneys.
- Simple brain without corpus callosum.