The amazing organisms that lay their eggs and feed their babies with milk are single-pass mammals. In our article we will consider the systematics and features of the life of this class of animals.
General characteristics of the class Mammals
Mammals, or Beasts, belong to the most highly organized representatives of the Chordate type. Their characteristic feature is the presence of mammary glands in females, the secret of which they feed their cubs. The external features of their structure include the location of the limbs under the body, the presence of hair and various derivatives of the skin: nails, claws, horns, hooves.
For most mammals, the presence of seven cervical vertebrae, the diaphragm, exclusively atmospheric breathing, a four-chamber heart, and the presence of a cortex in the brain are also characteristic.
Single pass, marsupials, insectivores: the origin of mammals
Mammals are characterized by significant species diversity. Platypus, kangaroo, mole, bat, dolphin, whale, monkey, man - all these are representatives of this class. All of them descended from ancient reptiles. The proof of this fact is the similarity of their embryonic development, the presence of cloaca and crow bones in some representatives, and egg laying.
As a result of evolutionary processes and further divergence, mammalian orders arose: monotremes, marsupials, insectivores. The origin of mammals, as well as their subsequent development, has led to the fact that at present this class occupies a dominant position in the system of the animal world. Its representatives have mastered both ground-air and aquatic habitats.
First Beast Subclass
This subclass of Mammals includes a single squad called One Pass. They got this name due to the presence of cesspools. This is one hole in which the ducts of the reproductive, digestive and urinary systems open. All these animals breed, laying eggs.
How can animals with such features be mammals? The answer is simple. They have mammary glands that open directly to the surface of the body, since single-pass do not have nipples. Newborns lick it directly from the skin.
Primitive features of the structure inherited from reptiles are the absence of cortex and gyrus in the brain, as well as teeth, the function of which are performed by horn plates. In addition, their body temperature fluctuates within certain limits, depending on its changes in the environment from +25 to +36 degrees. Such warm-bloodedness can be considered quite relative.
Single-pass egg laying cannot be called real. Often it is called an incomplete live birth. The fact is that the eggs do not immediately leave the reproductive ducts of the animal, but linger there for a certain time. During this period, the embryo develops already by half. After exiting the cloaca, single-pass hatches of eggs or carry them in a special leather bag.
Single Pass Mammals: Fossil Species
Paleontological findings of single pass are few in number. They belong to the Miocene, Upper and Middle Pleistocene eras. The oldest fossil of these animals is 123 million years old. Scientists have come to the conclusion that fossil remains practically do not differ from modern species. Single-pass mammals, representatives of which are endemic, live only in Australia and on the adjacent islands: New Zealand, Guinea, Tasmania.
Echidna
First Beasts is a group of animals that is represented by just a few species. Single pass mammals include echidna. Due to the fact that her body is covered with long stiff needles, outwardly this animal resembles a hedgehog. In case of danger, the echidna turns into a ball, thus protecting itself from enemies. The body of the animal has a length of about 80 cm, its front part is elongated and forms a small proboscis. Echidnas are nocturnal predators. In the afternoon they rest, and with the onset of dusk they go hunting. Therefore, their vision is poorly developed, which is offset by excellent sense of smell. Echidna have burrowing limbs. With the help of their and sticky tongue, they produce invertebrates in the soil. Females usually lay one egg, which is hatched in the skin fold.
Passageways
They are also representatives of the class Mammals, the order One-pass. They differ from their nearest echidna relatives by a longer elongated proboscis, as well as by the presence of three fingers instead of five. Their needles are shorter, most of them are hidden in the wool. But the limbs, on the contrary, are longer. Passages are endemic to the island of New Guinea.
The basis of the diet of these single-pass are earthworms and beetles. Like echidna, they catch them with a sticky long tongue, on which there are numerous small hooks.
Platypus
This animal as if borrowed its body parts from other representatives of this kingdom. The platypus is adapted to a semi-aquatic lifestyle. His body is covered with dense thick hair. It is very tough and practically waterproof. This animal has a beak and a tail of a beaver. On the fingers are swimming membranes and sharp claws. Males on the hind limbs develop horn spurs, into which the ducts of toxic glands open. For humans, their secret is not fatal, but can cause severe swelling of a certain area, and then the entire limb.
No wonder the platypus is sometimes called the "joke of God." According to legend, at the end of the creation of the world, the Creator left unused parts from different animals. Of these, he created the platypus. He is not just an Australian endemic. This is one of the symbols of the continent, the image of which is found even on the coins of this state.
This mammal hunts well in water. But it builds nests and burrows exclusively on land. This cute animal is not harmless. He swims at a significant speed, and grabs prey almost instantly - for 30 seconds. Therefore, aquatic animals have very few chances to hide from a predator. Thanks to valuable fur, the number of platypus has significantly decreased. Hunting for them is currently prohibited.
Subclass Real Animals
Single pass mammals are primarily characterized by the presence of cloaca. Real animals have separate openings for the digestive, reproductive, and urinary systems. In this subclass, marsupial and placental mammals are isolated.
Squad Marsupials
Representatives of this systematic unit have a leather bag on their belly. Some single pass mammals also have this feature. But at marsupials in it ducts of mammary glands open. Most of these animals live in Australia, but the possum is also found in North America.
The most famous representative of the order Marsupials is a kangaroo. This is a large mammal that moves in leaps. Their length can reach up to 1.5 m. Thanks to the well-developed hind limbs and tail, they move very quickly. Kangaroos can reach speeds of up to 50 km / h. These herbivores are often attacked by various predators. They are protected by the hind limbs, resting on the tail.
A marsupial bear, also called a koala, lives in southern Australia. This cute animal sits motionlessly in the trees all day. And at night he goes to an active lifestyle. The diet of koalas consists of leaves and young shoots of eucalyptus. These animals are quite voracious. They can eat up to a kilogram of food per day. The meat of koalas is inedible, but fur is of great value to humans. For this reason, this species was almost endangered. At this time, this animal is listed in the International Red Book.
Marsupials have mastered several habitats. Most of them are terrestrial animals. Some live in trees. This is a koala and a marsupial flying squirrel. Some species live underground. These include marsupial mole and possum.
Placental mammals
Mammals, monotremes, and marsupials are dioecious animals with internal fertilization. The most progressive structural features are placental representatives of this class. They are most prevalent in nature. During embryonic development, they form a child's place or placenta. This is the organ that provides the connection of the embryo with the mother's body. The placental pregnancy period is from 11 days in mouse-like rodents to 24 months.
This group of mammals is represented by a large number of orders. So, the representatives of Insectivores are hedgehogs, moles, muskrats, shrews, shrews. Their common feature is not only the nature of the food, but also the appearance. The anterior part of the head of the insectivore is elongated and forms a short proboscis, on which there are sensitive hairs.
Placental mastered all habitats, except the organism. Bats are capable of flying due to the presence of a skin fold between the fingers, which serves as their wing. The pinnipeds spend most of their lives in the water, and the cetaceans live there constantly. Terrestrial placental include Rodents, Hares, Pairs - and Equids, Predators and Primates. The man represents the last squad.
Mammals - monotremes, marsupials and placental feed their young with milk. Each of these superclasses has its own characteristics. In One-pass, cesspool is preserved, in marsupials, a skin fold is formed in which a newborn develops for a certain period. All of them are endemic to Australia. Marsupials and single pass mammals do not have a placenta. Due to the presence of an organ that binds the body of the mother and child during the period of intrauterine development, quite viable individuals are born. Therefore, placental are the most highly organized representatives of the class.