Mammals are chordates, and subtypes are vertebrates. In turn, there is a differentiation into two subclasses and several orders, which are divided into families.
The selection 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 "milk", which means "milk".
The mammary glands are evolutionarily derived from the sweat glands, but are more complex in comparison with them. These glands secrete a secret - milk, which contains water and three components of nutrients: proteins, fats and carbohydrates.
Mammal Subclasses
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:
- Oviparous.
- Placental
The first subclass has three names: ovipositing, single pass, primitive. The second subclass is divided into two infraclasses:
- Lower placental (marsupials).
- Higher placental.
Single pass
Single pass mammals are endemic that inhabit Australia, Tasmania and New Guinea. The subclass is represented by three representatives: platypuses, echidna and prochidina. These animals are not viviparous, therefore, the sign of live birth to all mammals will not apply. This symptom is characteristic only for placental. First animals lay eggs and feed offspring with milk. Platypuses hatch their eggs like birds, and the echidna wears them in a brood bag.
The structure of the mammary glands of single pass
In single-pass, the mammary glands look like paired elongated bags, inside the bag is a tube that is expelled by smooth muscles. The secret flows down the coat, as the nipples are reduced, and licks offspring. The name "one-pass" comes from the fact that their genitourinary sinus and intestines flow together into the cloaca. Hence their other collective name - cloacal.
Placental
The mammary glands of placental mammals are more complex. The section looks like lobed formations with complex branching ducts. The ducts end with a small area of the skin - the nipple.
Nipples are divided into two groups:
- False.
- The true ones.
Inside the false nipples there is one common channel, and in true nipples, 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 inguinal region.
The intensity and development of the mammary glands is associated with pregnancy and lactation, that is, the period of secretion and direct feeding of the offspring.
Placenta
To understand the essence of the systematics of placental, you need to determine what the placenta is. The placenta is the formation of chorionic villi fused together and connected with the walls of the uterus, that is, a special organ that communicates between the female’s body and the fetus during fetal development. Depending on the type of villi, the types of placenta are also distinguished:
- Vitelline.
- Allantoic.
In marsupials, the vitelline placenta is predominantly. The higher ones either have vitelline at first, which is later replaced by allantoic, or they function initially together.
The functions of the placenta:
- Protective. Do not miss an infection.
- Breathing
- Transport. Blood circulation occurs.
- Endocrine. The release of hormones.
Etc.
For a long time, there was an opinion that placental mammals evolved from monotremes, which is a mistake. Evolutionarily, these two subclasses appeared and developed independently of each other.
Only in placental mammals does a special fleshy mass appear around the mouth opening - lips.
Marsupials
Marsupial mammals (lower placental) give birth to underdeveloped cubs that are carried in the bag. The female herself licks the so-called “path” in the belly’s wool, along which the cub will move from the genital opening to the bag, where it is attached to the nipple.
Thus, based on the foregoing, we can conclude that the first sign in the differentiation of all mammals is the presence of the placenta or its absence (presence of cloaca). On this basis, the class of mammals was divided into two large taxa — a subclass.
Higher placental
Infraclass higher placental is divided into many units. The first sign of their differentiation is the structure of the dental apparatus. Another symptom emerges from this sign - the nature of the food. The sign of the structure of the dental apparatus is the second in mammalian taxonomy 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 lump in the mouth, that is, the main function of mammalian teeth is grinding food. In the remaining classes of chordates, the teeth serve to dismember or kill the prey. Consider the main units identified by this attribute:
Toothed
Families: sloths, armadillos, anteaters. These animals were assigned to the squad with the same name on the basis of the underdevelopment of the dental system. Their teeth are either enamelless or missing. For sloths, teeth are only pre-rooted and molar. Anteaters have completely missing teeth, there is a long and sticky tongue, thanks to which anteaters perfectly catch ants and termites.
Rodents
It 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 self-sharpen when the rodent nibbles something. If the animal does not gnaw, then it will simply die from the rupture of the jaw apparatus, thanks to very large incisors.
- Incisors have no roots.
- On the front side, the enamel layer is thicker.
- There is a special space between the molars and incisors - the diastema.
Forest representatives: squirrels, chipmunks and so on. Inhabitants of the soil are mole rats that make moves thanks to incisors. The largest representative of the world fauna is capybara. In the fauna of the temperate climate, the largest rodent is the beaver. The beaver is a typical phytophage, that is, eating plant foods. A rat is, so to say, a universal rodent, because it gnaws at everything, including concrete and iron.
Hare
Until the 50s of the twentieth century did not stand out at all. All animals of this detachment were assigned to the detachment of rodents. Later it was found that they had not one, but two pairs of incisors on the upper jaw. One is in front, the second is in back.
Predatory
The detachment is characterized by the presence of 4 incisors and two large fangs. Well-developed fangs are most developed in the extinct saber-toothed tiger. Representatives eat animal food. The following families are of the greatest importance: bear, marten, cat, and wolf. The largest land predator is the polar bear. Bears, unlike wolves, are stop-moving, that is, the emphasis falls on the whole foot as a whole. In addition, like all mammals, the nervous system is well developed, which makes behavior difficult. This is especially clearly seen in predators: the game appears in newborns, and it is an option for future hunting.
Insectivores
The teeth are small and sharp, the main food is insects. Main families: hedgehogs, moles, shrews.
Cetaceans
The sign of the cetacean tooth apparatus stands out clearly when two suborders are considered: 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 filtrators. Of the representatives, one can name the blue whale, which is the largest mammal in the world, and the bowhead whale.
Toothed whales, such as sperm whales, capture prey with conical teeth.
Aardvark
The order includes only one species - African aardvark. Teeth are only molar, not enameled. They look like fused tubules.
Proboscis
Their dental apparatus has a special education - tusks. These are overlying and protruding from the oral cavity upper paired incisors that grow throughout life. There is one molar on each side of the jaw; when worn, they are replaced by the following.
Sirens
Aquatic mammals, like cetaceans, have an amazing sign in the structure of the spinal column. In all mammals, the cervical spine consists of 7 vertebrae, and in sirens it consists of 9. Molar teeth with a flat chewing surface.
The detachment includes two families: dugongs and manatees. This group also included an extinct animal - a Steller's cow.
The third sign in mammalian taxonomy is the morphological structure of the limbs. This feature is the main one in the differentiation of two orders: artiodactyl and artiodactyl.
Artiodactyls
The extremities are four-fingered: the third and fourth fingers are longer, the second and fifth are much smaller.
Ungulate
The third finger is most developed.
All ungulates are finger-walking, which gives a good opportunity to run away from danger.