Invertebrate animals are numerous representatives of the fauna that do not have a spine. This category includes protozoa (unicellular), lower worms, sponges, arthropods, mollusks, echinoderms, and others. There are sixteen types in this group.
The division of the animal world into vertebrates and invertebrates was carried out by Lamarck in 1801. Among vertebrates, about forty-five thousand species are known. Known invertebrate animals are represented in the amount of 1 million 260 thousand species. In this case, the simplest - about twenty-five thousand, sponges - about five, lower worms - about twenty, mollusks - more than one hundred, arthropods - about seventy-nine thousand species.
The largest number of species of insects. There are about a million of them. At the same time, aquatic insects belong to many orders. It should be noted that some of them are present in water only during the larval period. While others are in it all their lives. Intestinal animals account for about nine thousand species. It should be noted that the existing types in nature are much more than is known. In addition, several thousand new ones are identified annually.
Invertebrate animals inhabit the soil, oceans, seas, rivers, lakes, ponds. Many of the representatives are parasites. At the same time they parasitize both on animals and on plants.
The role of invertebrates in nature is quite large. The solid remains of representatives who lived in the early geological epochs became elements of rocks, and in some cases their main mass. For example, the skeletons of invertebrate animals, long extinct, are almost entirely limestones.
The importance of the representatives of this group for a person is diverse. So, many invertebrate animals, as well as their metabolic products, are used for cooking for humans (bee honey, for example). Many representatives are used to feed commercial fish, animals, birds. Pearls, mollusk shells, beeswax and many other vital products of representatives of the invertebrate animals group are of great economic and technical importance. Often they are used in pest control. Invertebrates are predators and parasites that destroy them. This method relates to biological methods of pest control. Geologists in determining the age of sedimentary rocks examine the remains of fossil invertebrates.
However, in nature there are quite a lot of harmful representatives of this group. Many invertebrates are carriers of parasitic and infectious pathologies, pests of grain and products from it. They can be poisonous, as well as damage crops, forests, etc.
Intestinal cavity occupy a special place in the group. The description traditionally indicates the presence of radial symmetry, as well as two germ layers (ectoderm and endoderm). Usually, it is believed that the animal’s body is formed from two epithelial layers: the outer integument (epidermis) and the lining in the intestinal cavity (gastroderma). Between the layers is a connective tissue jelly-like layer (mesogley). It contains collagen fibers and amoeboid cells in small quantities.
Representatives of the intestinal cavity lack specialized respiratory and excretory organs . This is usually associated with a two-layer structure structure, which provides for the contact of most epithelial cells with the external environment.
The intestinal cavity is characterized by an underdeveloped nervous system. Most reproduces sexually and is characterized by the presence of crawling or planktonic larvae.