According to the theory of evolution, all kinds of living beings on Earth gradually, over long millions of years, evolved from their single-celled ancestors. More complex organisms most likely arose from colonies of protozoa. This can be tracked if we study the main types of animals in more detail. The classification divides all creatures into species, families, orders, classes according to their structure and external features, which were acquired during evolutionary improvement.
New types of animal tissues were formed, organs appeared that were not in the most ancient ancestors. The initial stage of such progress can be observed in sponges. The coelenterates already have well-defined endoderm and ectoderm, as well as the rudiments of muscles. The higher types of animals are characterized by a 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 one-celled structure. Scientists know about 15 thousand species of protozoa. The shape of their body is different, from radiant-radial to asymmetric. They often form complex colonies, which allows scientists to speculate about how multicellular types arose.animals. They are divided into classes, depending on the methods of movement and body structure.
Sponges
The most primitive multicellular organisms. They live mostly in the sea. They are divided into 3 classes, depending on the composition of the skeleton. They have a fixed lifestyle. Other types of the Animal Kingdom are opposed to them because sponges lack characteristic organs and tissues. There is an outer layer that protects the body from the surface, and an inner layer consisting of special flagellar collar cells. Between them is the mesoglea - sometimes a very massive group of cells, some of which form the skeleton.
Celiac
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 muscular tissue. There is no circulatory and excretory system. The lifestyle of the intestinal cavity is sedentary or free-moving. They live, with rare exceptions, in sea water and form extensive colonies. This type includes jellyfish, corals, hydroid polyps and sea anemones.
Flatworms
Planet-bodied creatures that have the rudiments of the excretory system and the brain. The anal opening is still missing. Representatives of this type are hermaphrodites. This type includes ciliary worms, or turbellaria, as well as some parasites - tapeworms and flukes.
Roundworms
They have a mouth and anus connected by an intestine. The main group is nematodes, among whichmany parasites, but there are also free-living species. This is a blind branch of evolution; this group did not have any further influence on the development of organisms. This type also includes hairy, rotifers and acanthocephalans, which are often considered as separate groups.
Annelled worms
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 a secondary body cavity. Other, more highly developed types of the Animal Kingdom were shaped by these changes. Numerous representatives of the arthropod group originated from marine annelids.
Shellfish
Animals whose soft body is usually protected by a shell. They have a highly developed nervous system, a secondary body cavity. The sense organs and the heart, the muscle that pumps blood, appeared. In bivalves, the body 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, thanks to which the five-ray symmetry peculiar only to echinoderms has developed. Representatives of the type have a circulatory system, a mesoderm internal skeleton.
Arthropods
Types of animals are very extensive. Such a group are Arthropods. This type- the most diverse and rich in species. Characteristic features of the type are the presence of complex sensory organs in the form of isolated appendages of the oral cavity - antennae, a clear division of the body into sections, limbs, consisting of segments, for more efficient movement. The development of arthropods passed from the extinct trilobites, a primitive group that is ancestral to crustaceans and arachnids, to higher flying insects. Centipedes are considered a transitional link in the evolution of this type.
Chordates
Type includes species and classes that are diverse in their appearance, lifestyle, 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 the numerous endings, which is protected by the chord, cartilage or bone rod, the support of the skeleton. The development of representatives of various classes can be traced from larval and non-cranial (lancelets) to complexly organized primates with high intelligence.
Pisces
There are cartilaginous, lobed or fleshy-lobed, bone. Representatives of the first group have dense skin with placoid scales peculiar only to them. The mouth is located on the underside of the body, there are no lungs and swim bladder, the skeleton consists of cartilage.
Lobe-finned fish are divided into lungfish and lobe-finned fish. 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 andlungs.
Bone - this is a large part of the modern representatives of the class of fish. They have a swim bladder and a hard skeleton; the skin is mostly scaly, with numerous exceptions.
Amphibians
As a rule, the larvae of these creatures breathe through gills and live in water. The adult has lungs and lives on land. The skin is hydrated 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 ancient times, this class dominated among the rest in terms of numbers, but after that the main place was taken by mammals. 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 their body temperature, regardless of environmental conditions. Birds have well-formed lungs, a four-chambered heart, and wings that 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 their young. The body is usually covered with hair, they are warm-blooded, the limbs are brought under the body and turned forward. The 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 waysupply:
• Herbivores. Eat exclusively plant foods - algae, herbs, leaves or fruits. For example, elk, deer, rabbit.
• Predators. They eat insects or the flesh of other animals. For example, frog, tiger, lynx.
• Omnivorous. Depending on the environmental conditions, they can eat both plant and animal foods. For example, bear, tit, wild 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, into fresh water or into underground caves. In connection with a radical change in the habitat, the types of animal tissues changed and improved, which was necessary for survival. Some groups - whales, reptiles and birds - then returned to the sea, having gone through a long evolutionary path.
Now most classes live in or near the ocean. So many animal species, especially invertebrates, remain unchanged for millions of years and are a valuable resource for study. Other major animal phyla are thought to be relatively young, but their study has helped uncover genetic links between seemingly distinct groups. This has a huge impact on understanding the unity of man with the surrounding nature and understanding the great similarity of living beings.
