Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Class Mammalia
Subclass Prototheria, or Monotremes

This subclass unites primitive, egg-laying mammals belonging to a single order, Monotremata, which is represented in the modern fauna by only a few species inhabiting Australia and adjacent islands. These animals reproduce by laying eggs containing a half-developed embryo. The embryo completes its development either during incubation (in platypuses) or while the egg is carried in a pouch (in echidnas). Egg-laying mammals lack nipples; the numerous ducts of their Mammary Glands open onto specialized Skin areas called glandular fields, which are devoid of Hair. When a glandular field is lightly pressed, milk oozes directly from the ducts to be licked off by the young. Prototherians lack fleshy Lips and Teeth. The Cerebral Cortex is poorly developed. Although a hair coat is present, thermoregulation is imperfect, and body Temperature is relatively low, fluctuating between 22 and 37°C.

The order Monotremata comprises two families: the platypuses and the echidnas (Fig. 60).

The platypus family includes only a single species, the platypus, native to Australia and the island of Tasmania. It is a semiaquatic animal covered with dense, coarse fur that remains waterproof for a long time. The front paws of the platypus feature webbed membranes and sharp, strong claws. Its snout resembles a duck's bill; however, unlike a rigid bird's beak, it is soft and covered with elastic skin stretched over two thin, elongated, arched bones. The skin of the bill is rich in nerve endings, which help the animal navigate and locate food in the Water. The platypus inhabits quiet bodies of water. It feeds on aquatic invertebrates (Mollusks, small crustaceans, insect larvae), which it captures by filtering water stirred up from the bottom. The platypus swims and dives well, but moves slowly on land. It builds its nest in a burrow with an underwater entrance. It lays two eggs, and outside the mother's body, incubation lasts only 9–10 days.

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Fig. 60. Egg-laying mammals:

1 — echidna; 2 — long-beaked echidna; 3 — platypus.

The echidna family includes two species, found in Australia as well as on the islands of Tasmania and New Guinea. These are specialized burrowing mammals whose limbs are armed with long, strong claws. Their bodies are covered with coarse hair and sharp spines resembling those of a hedgehog. The snout features a relatively short, cylindrical beak. Echidnas live in burrows across hilly terrain, preferring areas with dense shrubbery. They feed on insects, which they Histology/2.html">EXTRACT FROM THE ground or from crevices between rocks using a long Tongue coated in sticky saliva. During the breeding season, the echidna lays a single egg on the ground, which it then transfers to a leathery abdominal pouch, incubating it for 10–11 days. The newly hatched young remains in the pouch for up to 2.5 months, until it reaches a length of about 9–10 cm. By this time, its spines begin to grow, making it very uncomfortable for the mother to carry. She then finds or digs a suitable burrow and deposits her offspring there, continuing to visit and feed it for quite some time. The family also includes long-beaked echidnas, with three known species inhabiting the island of New Guinea. They differ from echidnas by having a longer and slenderer beak, and share a similar lifestyle.