Mammals are phylum chordates, subtype - vertebrates. In turn, there is a differentiation into two subclasses and several orders, which are divided into families.
The classification of the class Mammals occurs according to one archival anatomical and morphological feature - the presence of mammary glands, feeding their offspring with milk. This feature gives this class independence from environmental conditions, that is, food for newborn offspring does not need to be sought and obtained. Based on this, the name of the class comes from the obsolete word "mleko", which means "milk".
The mammary glands are evolutionarily derivatives of the sweat glands, but in comparison with them, they are more complex. These glands secrete milk, which contains water and three nutrient components: proteins, fats and carbohydrates.
Due to the fact that mammals have a rather complex anatomical and morphological structure of the genital organs and a fundamental difference in the methods of reproduction, in zoological systematics they are divided into two subclasses:
The first subclass has three names: oviparous, monotreme, first beasts. The second subclass is subdivided into two infraclasses:
- Inferior placentals (marsupials).
- Higher placental.
Single pass mammals are endemic to Australia, Tasmania and New Guinea. The subclass is represented by three representatives: platypuses, echidnas and prochidnas. These animals are not viviparous, so the sign of live birth will not apply to all mammals. This sign is characteristic only for placental. The first animals lay eggs and feed their offspring with milk. Platypuses incubate their eggs like birds, while echidnas carry them in their brood pouch.
Structure of monotreme mammary glands
In monotremes, the mammary glands look like paired elongated sacs, inside the sac is a tube that is expelled by smooth muscles. The secret flows down the coat, as the nipples are reduced, and is licked off by the offspring. The name "single pass" comes from the fact that their urogenital sinus and intestines flow together into the cloaca. Hence one more of their collective name - cesspools.
The mammary glands of placental mammals are more complex. In the context, they look like lobed formations with complexly branching ducts. The ducts end in a small area of skin - the nipple.
Nipples are divided into two groups:
Inside false nipplesthere is one common channel, while in true ones each duct passes independently.
The number of mammary glands can vary from 2 to 26 depending on the type of mammal. In addition, their location is different. For example, in primates they are located on the chest, in ungulates - in the groin.
The intensity and development of the mammary glands is associated with pregnancy and lactation, that is, the period of secretion and direct feeding of offspring.
To understand the essence of placental taxonomy, you need to define what a placenta is. Placenta - the formation of chorionic villi, fused together and connected with the walls of the uterus, that is, a special organ that communicates between the body of the female and the embryo during intrauterine development. Depending on the type of villi, types of placenta are also distinguished:
Marsupials predominantly have a yolk placenta. In the higher ones, either the vitelline system functions first, later replaced by the allantoic one, or they initially function together.
Functions of the placenta:
- Protective. Does not pass infections.
- Transport. There is blood circulation.
- Endocrine. Release of hormones.
And so on.
For a long time it was believed that placental mammals evolved from monotremes, which is wrong. Evolutionarily, these two subclasses appeared and developed independently of each other.friend.
Only placental mammals have a special fleshy formation around the mouth opening - lips.
Marsupial mammals (lower placental) give birth to underdeveloped cubs, which are worn in a bag. The female herself licks the so-called "path" in the fur of the belly, along which the cub will move from the genital opening to the bag, where it sticks to the nipple.
Thus, based on the above, we can conclude that the first sign in the differentiation of all mammals is the presence of a placenta or its absence (the presence of a cloaca). On this basis, the class of mammals was divided into two large taxa - subclasses.
Infraclass higher placental is subdivided into many orders. The first sign of their differentiation is the structure of the dental apparatus. From this sign comes another sign - the nature of the food. The sign of the structure of the dental apparatus is the second in the taxonomy of mammals after the sign of the presence of the placenta.
It should be noted that mammals are the only class of chordates that create a food bolus in the mouth, that is, the main function of mammalian teeth is grinding food. In other classes of chordates, the teeth are used to dismember or kill prey. Consider the main units identified on this basis:
Families: sloths, armadillos, anteaters. These animals were identified in a detachment with this name on the basis of underdevelopment of the dental system. Their teeth are either devoid of enamel ormissing. Sloths have only premolar and molars teeth. Anteaters have no teeth at all, have a long and sticky tongue, thanks to which anteaters are excellent at catching ants and termites.
Includes a large number of families (about 32). All rodents are united according to the following features of the dental system:
- The presence of one pair of incisors, growing throughout life, which must be constantly grinded. The incisors sharpen themselves when the rodent chews on something. If the animal does not gnaw, then it will simply die from a rupture of the jaw apparatus, thanks to very large incisors.
- Incisors have no roots.
- The enamel layer is thicker on the front side.
- There is a special space between the molars and incisors - the diastema.
Forest representatives: squirrels, chipmunks and so on. The inhabitants of the soil are mole rats, who make moves thanks to their incisors. The largest representative of the world fauna is the capybara. In the fauna of the temperate climate, the largest rodent is the river beaver. The river beaver is a typical phytophage, that is, it feeds on plant foods. The rat is, so to speak, a universal rodent, as it gnaws through everything, including concrete and iron.
Until the 50s of the twentieth century, it did not stand out at all. All animals of this order were classified as rodents. Later it was found that they have not one, but two pairs of incisors in the upper jaw. One is in the front, the other is in the back.
The detachment is characterized by the presence of 4 incisors and two large fangs. Well developedfangs reached their greatest development in the extinct saber-toothed tiger. Representatives eat animal food. The following families are of the greatest importance: bear, marten, cat, wolf. The largest land predator is the polar bear. Bears, unlike wolves, are plantigrade, that is, the emphasis falls on the entire foot as a whole. In addition, like all mammals, the nervous system is well developed, which makes it difficult to behave. This is especially evident in carnivores: newborns have a game, and it is a variant of the future hunt.
The teeth are small and sharp, the main food is insects. Main families: hedgehogs, moles, shrews.
The sign of the dentition in cetaceans stands out well if we consider two suborders: baleen whales and toothed whales.
Baleen whales have a special formation - a whalebone, which traps plankton in the manner of a filter. The beak of a duck is built on the same principle. That is why baleen whales are called filter feeders. Representatives include the blue whale, which is the largest mammal in the world, and the bowhead whale.
Toothed whales, such as the sperm whale, capture prey with conical teeth.
The squad includes only one species - the African aardvark. Teeth only molars, not covered with enamel. They look like fused tubules.
Their dental apparatus has a special formation - tusks. These are overgrown and protruding from the mouthcavities are the upper paired incisors that grow throughout life. There is one molars on each side of the jaw, when they wear out they are replaced by the following.
Aquatic mammals, like cetaceans, but have an amazing feature in the structure of the spinal column. In all mammals, the cervical spine consists of 7 vertebrae, and in sirens - from 9. Molar teeth with a flat chewing surface.
The order includes two families: dugongs and manatees. An extinct animal, the Steller's cow, also belonged to this order.
The third feature in the taxonomy of mammals is the morphological structure of the limbs. This feature is the main one in the differentiation of two orders: artiodactyls and equids.
The limbs are four-fingered: the third and fourth fingers are longer, the second and fifth are much smaller.
The third finger is the most developed.
All ungulates are digitigrade, giving them a good opportunity to run away from danger.