According to the theory of evolution, all kinds of living things on Earth gradually, over millions of years, evolved from their unicellular ancestors. More complex organisms most likely arose from protozoa colonies. This can be traced if we study in more detail the main types of animals. Classification divides all creatures into species, families, orders, classes according to their structure and external characteristics, which were acquired during evolutionary improvement.
New types of animal tissues were formed, organs appeared that were not found in ancient ancestors. The initial stage of such progress can be observed in sponges. The intestinal cavity already has a pronounced endoderm and ectoderm, as well as the beginnings of muscle. Higher types of animals are characterized by the complex structure of the nervous system and other organ systems. To understand evolution, it is necessary to consider in more detail their most important features.
Protozoa
These are microscopic creatures with a unicellular structure. Scientists know about 15 thousand species of protozoa. Their body shapes vary from radial to asymmetric. Complex colonies are often formed, which allows scientists to suggest how multicellular types of animals arose. They are divided into classes, depending on the methods of movement and body structure.
Sponges
The most primitive multicellular organisms. Mostly live in the sea. They are divided into 3 classes, depending on the composition of the skeleton. Their lifestyle is attached. Other types of the Animal Kingdom are opposed to them, because the sponges lack characteristic organs and tissues. There is an outer, protecting the body from the surface, and an inner layer consisting of special flagellar collar cells. Between them is a mesogley - sometimes a very massive group of cells, some of which form a skeleton.
Coelenterates
The bodies of these animals consist of only two layers of cells that surround the body cavity, called the intestinal, with one mouth opening. They have the rudiments of nervous and muscle tissue. There is no circulatory and excretory system . The lifestyle in the intestinal can be sedentary or free-moving. They live, with rare exceptions, in seawater and form vast colonies. This type includes jellyfish, coral, hydroid polyps and sea anemones.
Flatworms
Flat-bodied creatures that have the rudiments of the excretory system and brain. The anus is still missing. Representatives of this type are hermaphrodites. This type includes ciliary worms, or turbellaria, as well as some parasites - lenticans and flukes.
Roundworms
They have oral and anal openings connected by intestines. The main group is nematodes, among which there are many parasites, but there are also free-living species. This is a blind branch of evolution; this group did not have a further influence on the development of organisms. This type also includes hairs, rotifers and scrubs, which are often considered as separate groups.
Annelids
The bodies of such animals consist of separate segments. They have a circulatory system, a high ability to regenerate the rudiments of primitive limbs and the secondary cavity of the body. Other, more advanced types of the Animal Kingdom were formed under the influence of these changes. Numerous representatives of the arthropod group originated from marine annelids.
Mollusks
Animals whose soft body is usually protected by a shell. They have a highly developed nervous system, a secondary body cavity. The sensory organs and the heart appeared - the muscle that pumps blood. In bivalves, the trunk and leg can be distinguished, in gastropods - the head. They live both in sea and fresh water, and on land.
Echinoderms
Inhabitants of the deep sea. The sizes of the largest representatives do not exceed 50 cm. The type includes classes of sea urchins, stars, lilies and others. The way of life is motionless, due to which five-beam symmetry, characteristic of only echinoderms, has developed. Representatives of the type have a circulatory system, a mesodermal internal skeleton.
Arthropods
Types of animals are very extensive. It is this group that arthropods are . This type is the most diverse and rich in species. Characteristic features of the type are the presence of complex sensory organs in the form of distinguished appendages of the oral cavity - antennae, a clear division of the body into departments, limbs consisting of segments, for more efficient movement. Arthropods developed from extinct trilobites, a primitive group that is ancestral to crustaceans and arachnids, to higher flying insects. Millipedes are considered a transitional link in the evolution of this type.
Chordate
The type includes species and classes that are diverse in their appearance, lifestyle, and habitat. Types of the nervous system in animals are united by a tube formed on the dorsal part of the body, which is the center of all numerous endings, which is protected by a chord, cartilage or bone rod, and skeleton support. The development of representatives of various classes can be traced from larval-chordal and disembodied (lancelet) to highly organized primates that are distinguished by high intelligence.
Fish
There are cartilaginous, lobe-steppe or meat-lobed, bone. Representatives of the first group have dense skin with a placoid, their only characteristic scales. The mouth is located on the underside of the body, there are no lungs and swim bladder, the skeleton consists of cartilage.
The lobatefin fish are divided into bipedal and brushback. The latter are now represented by only one genus living in the Indian Ocean. They are very similar to the ancestors of amphibians and are of particular interest to researchers who support the theory of evolution. Lungfish have both gills and lungs.
Bone - this is most of the modern representatives of the fish class. Have a swimming bladder and a solid skeleton; the skin is mostly covered with scales, but there are numerous exceptions.
Amphibians
As a rule, the larvae of these creatures breathe with gills and live in water. An adult has lungs and lives on land. The skin is moisturized and devoid of hair or scales. This class includes frogs, newts, toads, salamanders.
Reptiles
The body is covered with scales; they live both on land and in water. In antiquity, this class dominated among the rest in numbers, but after that mammals took the main place. They have a variety of sizes, body shape, lifestyle. Crocodiles, lizards, snakes, turtles are representatives of reptiles.
Birds
Anatomically close to reptiles, but they have the ability to independently maintain the temperature of their body, regardless of the environment. The birds have excellently formed lungs, a four-chamber heart and wings, which allow most of them to move through the air.
Mammals
They are named so because of the presence of special glands, the secret of which they feed the cubs. The body is usually covered with wool, they are warm-blooded, the limbs are brought under the body and turned forward. Higher mammals, primates, develop intelligence, which is very conducive to survival.
Types of animal nutrition
All creatures are divided into 3 categories according to the method of nutrition:
• Herbivores . They use exclusively plant foods - algae, herbs, leaves or fruits. For example, elk, deer, rabbit.
• Predators . Eat insects or the flesh of other animals. For example, a frog, a tiger, a lynx.
• Omnivores . Depending on environmental conditions, they can eat both plant and animal food. For example, bear, tit, boar.
Ocean of life
The ancient ancestors of modern creatures gradually emerged from the ocean, which became the cradle of life on Earth. This migration could take place in several ways - across the coast to land, to fresh water, or to underground caves. In connection with the cardinal change in the environment, the types of animal tissues changed and improved, which was necessary for survival. Some groups — whales, reptiles, and birds — then returned to the sea, following a long evolutionary path.
Representatives of most classes now live in or near the ocean. Many animal species, especially invertebrates, remain unchanged for millions of years and constitute a valuable resource for study. Other main types of animals are considered relatively young, but their study helped to identify genetic relationships between seemingly different groups. This has a huge impact on the awareness of the unity of man with the environment and understanding the enormous similarity of living beings.