Brief introduction of existing protozoa and postprotozoa
At present, the classification of mammals in the industry is mainly divided into three subclasses: protozoa, post-mammals and true animals. Everyone is familiar with the subclass Euthera. Let me briefly talk about the existing animal representatives of the first two subclasses: 1. Protozoa include the most primitive mammals in existence. Many primitive features similar to those of reptiles are still preserved, mainly in the following aspects: laying eggs, the eggs laid are eggshells, rich in yolk, large in diameter, 14mm (generally, the diameter of mammalian eggs does not exceed 0.2mm), females still have the habit of incubating eggs (platypus), or feeding eggs with special breast bags (echidna). Male testicles are located in the body, and there is no scrotum outside. There is a small penis, which is located in the cloaca, much like a turtle's penis, except that the groove on its back becomes a tube. There is a cloaca, which is opened out of the body with a single cloaca hole (so it is also called a single hole). The mouth has a flat beak and no fleshy lips. No outer ear shell. Adults have no teeth in their mouths, and larvae have temporary nodular teeth, which will not produce permanent teeth after falling off. Shoulder straps are reptiles, with independent black coracoid, anterior black coracoid and developed clavicle. There are a pair of pouches (or pubis) on the belt. The cerebral cortex is underdeveloped, and there is no corpus callosum between the two hemispheres. On the other hand, monotremes also have the characteristics of mammals, and they are already primitive mammals. For example, there are hairs on the body surface and mammary glands, which are specialized sweat glands without nipples. The mammary duct opens in the mammary region of the abdominal wall, and young animals lick milk. Muscular diaphragm. Only the left arterial arch. The mandible consists of a single tooth bone. The body temperature fluctuates between 26 ~ 35℃, and it is in the transition stage from variable temperature to constant temperature. There is only one order in this subclass, that is, Monoptera, which is only distributed in Australia and its nearby islands and is divided into 2 families: platypus: only one genus and one species exists, that is, platypus, a world-famous rare animal. Platypus is a semi-aquatic animal, which has a series of characteristics to adapt to aquatic life. He is covered with short, dense brown hair and won't stay in the water for a long time. The mouth is wide and flat, with no fleshy lips and horny sheaths. There are notches on both sides of the mouth to filter food. It looks like a platypus, hence the name platypus. The tail is flat and wide, webbed between the four toes, and webbed forelimbs are particularly developed. They are good at swimming or diving. Feeding on mollusks, crustaceans, worms and aquatic insects. Living by the river, one end of its cave opens into the water, and the other end expands into a nest on the shore. Mating in the water and incubating eggs in the nest. It was not until the19th century that people discovered that this small beast with a flat mouth was actually laying eggs! It lays two eggs per nest, and the incubation period is 14 days. The hatched cubs are bare and hairless, grow up by feeding, and live independently after 4 months. The platypus represents the transitional stage from reptiles to mammals, and it is the most precious "living fossil". Engels said in Anti-Turin that "we now know that there are mammals that hatch eggs" refers to platypus. There are 2 genera and 3 species of echidna, namely, echidna and protoechidna. The body surface of the echidna is needle-shaped, with coarse hairs between the spines and only coarse hairs on the ventral surface. It looks like a hedgehog and can curl up. A long tubular kiss with nostrils open at the end. The upper and lower jaws have no teeth, but they have a long retractable tongue with developed salivary glands. They feed on ants and insects, and their tongues are full of saliva. Small eyes, no outer ear shell and short tail. The front and rear limbs have five toes and claw tips, which are suitable for digging ant nests. The echidna can dig holes at an amazing speed and bury itself in the soil. Every time a female animal lays 1-2 eggs with leathery shells, they hatch in the abdominal skin sac, which is temporary during the breeding period, protruding out of the belt and supported by the upper part of the pubic bone (bag bone). The incubation period is 28 days, and the larvae hatch under the condition of incomplete development, feeding on the milk secreted by the mammary gland in the skin capsule and continuing to develop. Photo of the Echinoceros The original Echinoceros (Zaglossus bruijnii), also known as the long-nosed Echinoceros, looks like the Echinoceros, but it bends down after kissing the minister. The middle three toes of the five toes of the front and rear limbs are particularly large, and the other two toes are degenerated. Now it is only distributed in New Guinea, but it was found in Pleistocene Australia and Tasmania. The protomole is almost twice as big as the mole, and it is the largest member of the monotremes. Its beak is long and curved, its thorns are short and sparse, and it has more hair. In the past, protozoans were divided into three different species, of which trichii was widely distributed on the island, and the other two were confined to the highlands in the northeast. At present, the long-nosed echidna are all classified into the same species and have different subspecies. Photo of the original mole rat II. Invertebrate suborder, also known as marsupials, is a kind of mammal that is between oviparous monotremes and advanced placenta on the evolutionary level. Its main characteristics are: viviparous, but most of them have no real placenta, and the mother beast has a special pouch. After birth, the immature baby needs to continue to develop in the maternal abdominal sac. There are nipples on the breast, and the nipples are open in the pouch. Young marsupials have well-developed forelimbs that are out of proportion to the rest of the body. The cubs use these thick forelimbs to crawl for nipples. The brain hemisphere of marsupials is very small, with no sulcus and corpus callosum. The body temperature is close to constant temperature and fluctuates between 33-35℃. Women have double uterus and double vagina. Correspondingly, the end of the male penis is also divided into two forks, and each fork enters a vagina during mating. Male scrotum is outside. The teeth are irregular, and the front teeth are numerous and diverse. The skeleton is close to that of a mammal with placenta: no neck rib, 13 thoracic vertebrae with ribs, usually 7 lumbar vertebrae. The coracoid bone, the front part of coracoid bone and clavicle in the shoulder strap have degenerated, and the scapula is enlarged with scapula ridge. The belt has an upper pubic bone (bag bone) to support the bag. This subclass is mainly distributed in Australia and its adjacent islands, with a few species in South America and Central America and only one species in North America. There is only one order in existence, namely marsupials. According to the number of front teeth, it can be divided into three suborders: multi-door
At present, there are more than 4,600 kinds of mammals living on the earth, which biologists divide into three categories: monotremes, marsupials and placentas. The number of single-celled mammals is very rare, with only three species, and the most famous representative is platypus. Placenta is a big family with more than 4300 species. In between, there are more than 270 marsupials. In biology, marsupials belong to the suborder Animalia, as opposed to the suborder Animalia to which placental animals belong. Subanima includes not only lovely kangaroos, koalas and marsupials, but also many marsupials that have become extinct in the long process of evolution. Jack, whose scientific name is Sinodelphys, is one of them and the oldest one.