Mammals are the most developed class in the animal kingdom. They are over 200 million yeas old and lived even at the age of the dinosaurs. As their name reveals they feed their young with mammary glands. They give birth to live young and have hair compared to feathers or scales which other animal classes have.
Anatomy: From the anatomical point of view, their skeleton is in many ways similar to the human body. Their body is covered with a fur. They have got a bony skeleton. The spine can be divided into cervical, thoracic, lumbar, sacral and a tail part. The cervical consists of 7 parts / vertebrae (just like the humans), thoracic of 12-18 vertebrae and lumbar of 2-8 vertebrae. Sacral vertebrae are together fixed in the sacral bone. The number of vertebrae in the tail part may vary a lot. The first two vertebrae (atlas and axis) enable mammals to move their head to the sides (left / right) as well as up and down. Ribs are fixed to the thoracic vertebrae. They are also used as a great protection.
Physiology: Lungs consist of many small alveoli - this rapidly increases their inner surface. Diaphragm is between the thorax and the abdominal part. It also participates in breathing and is an inevitable inhaling muscle. The heart is made of four completely separated parts. There are two atriums and two ventricles. Deoxygenated blood flows through the right heart whereas the oxygenated blood flows through the left heart. So there are two separated blood circulations. The blood circulation starts in the right atrium and then the blood flows into the right ventricle. From the right ventricle the blood flows into the lungs where it is oxygenated. Consequently it flows into the left atrium and then into the left ventricle. From the left ventricle the oxygenated blood flows into the entire body to all organs. There it gives inevitable oxygen to tissues and last but not least the deoxygenated blood comes through veins into the right atrium. And in the right atrium it starts all over again. Mammals have got a constant body temperature, they are endothermic. They also do have sweat glands which enable them to cool themselves.
The alimentary canal starts with a mouth with teeth. Teeth have got different shape and different functions too. A food comes through the pharynx and esophagus into the stomach (gaster). Chemical digestion starts in stomach and is finished in duodenum, jejunum and ileum. Pancreas and gall bladder participate in digestion. Gall helps to digest fat and pancreatic enzymes proteins and sugars. Intestunum tenue (duodenum, jejunum and ileum) participates (apart from digestion) in resorption too. The concentration of indigestible food takes place in colon. Only water is digested there. The alimentary canal ends as an anus.
Mammals have got pair of kidneys, which excrete toxic and abundant fluid. They are placed in the lumbar area on the both sides. Nervous system is extraordinary developed. The frontal part has got the most impact over other brain parts. Sensory organs are well developed as well.
Reproduction: There is always an internal fertilisation. Development of a new young takes place in the female’s organ called uterus. During the intrauterine development the small young is/are connected to the mother’s body with placenta. The placenta fulfils plenty of functions. Firstly it enables breathing, then nutrition and last but not least it excretes toxic substances. Intrauterine period is finished by giving birth. After the birth, a small offspring breaths an atmospheric air with its lungs and starts to suck mother’s milk (=mammals).
Systematically we can divide Mammalia Class into two Suclasses: Prototheria (Monotremes) and Theria Subclass. Consequently Theria Subclass is divided into two other Infraclasses: Metatheria/Marsupialia Infraclass (Marsiupial mammals) and Eutheria/Placentalia Infraclass (Placenal mammals).
Prototheria (Monotremes) are primitive mammals. They live in Australia and nearby islands. They still have got some features reminding of birds. Jaws are prolonged. Alimentary tract, secretory and sex organs have the same end called cloaca. They lay eggs and small ones suck mother’s milk, however nipples are not developed. This Subclass includes animals such as Ornithorhynchus and Echidna (Spiny anteater).
Metatheria/Marsupialia Infraclass (Marsiupial mammals) includes animals for instance Macropus (kangaroo), Didelphis, Notoryctes, Petaurus, Phascolaretos (koala). Females have got 2 uteruses and a special sack (marsupium) on the abdomen. There is an underdeveloped placenta in the uterus and cannot enable the full foetus development. Therefore Marsupial mammals give birth to immature small young. Their development continues and is finished in a unique marsupial pouch outside the mother’s belly which also contains nipples.
Placentalia Infaclass include a huge number of animals. They are spread all around the world. They create a placenta, which enables foetus a complete intrauterine development. This infraclass can be also divided into several subcategories. Insectivora have teeth which are underdeveloped. Their brain is very small and hemispheres are smooth and do no contain any gyruses. Their food mainly consists of insects. This group includes Erinaceus (hedgehog) and a Mole (Talpa). Chiroptera (Bats) are mammals which are able to fly! Their body structure is ideal for flying (strong clavicles, long fingers etc.). Their sense of hearing is developed exceptionally well. They move by using echolocation. On the other hand their sight is very poor. Edentata: these animals live in South America. Their teeth are very weak. Rodentia: their jaws are very strong and include sharp teeth. They breed several times per year and have plenty of small young. These young mature rather early. This group includes a mouse (Mus), a rat (Ratus) etc. Carnivora have very strong jaws with knife-sharp teeth. This group includes animals such as a cat (Felis), a lion (Felis), a jaguar (Panthera), a tiger (Felis), a dog (Canis), a fox (Vulpes), a bear (Ursus), hyena (Hyena) etc. Pinnipedia is another group. These animals live in water (for instance: a seal (Phoca). Whales (Cetacea) also live in water. They have got a huge amount of fat located under the skin. It is a great isolation layer. This group can be divided into two subgroups: whales with teeth (e.g.: Balaena) and whales without any teeth (e.g.: a dolphin (Delphinus)). Horses (Equus), rhinos (Rhinoceros) belong to Perissodactyla group. Artiodactyla have only four fingers. The first finger (thumb) is not developed. Almost the entire weight of the animal is on the third and the fourth finger. Some animals from this group have got only a simple stomach (Hippopotamus (Hippopotamus)) whereas other animals have got a stomach made of four parts (e.g.: a bison (Bison), a deer (Cervus), a camel (Camelus) etc.). Elephants belong to Proboscidae.
Primates (Primates) have got five fingers on each extremity. The thumb can oppose other fingers which enable to grab things. Some primates have nails others claws. Lemurs (Lemur), Monkeys (Simiae) have got nails. Their brain is well developed then hemispheres are big and contain gyruses. Some animals have a wide nose septum (Hapalidae) whereas others a thin one (Catarrhina). Others have only a subtle nose (e.g.: Gorillas (Gorilla), Orang-utans (Pongo)).