Invertebrates

There are about 92 thousand species of invertebrates in the world: these are arachnids, mollusks, worms, echinoderms, sponges - all the variety of animals that do not have a backbone. But one should not think that their species are poor. This type of body organization is the most diverse in the animal world, as we will see from this article.

Invertebrates constitute the order "Arthropods" - animals with a segmented body and jointed limbs. Therefore, many invertebrates seem quite similar to each other. In particular, all invertebrates have simple transparent eyes on a stalk, a mouth with tentacles or teeth in the case of lobsters or sharks, gills in the cases of ascidians, internal fertilization in the cases of pulmonate molluscs or coral polyps. They are also similar in that they have passages and chambers filled with jelly-like tissue - for example, ganglia. And the whole invertebrate kingdom is united by the fact that they all have ring vessels in the circulatory system, calcareous compounds in the bones and carbon dioxide as the main gas. More than a million (one million two hundred sixteen thousand one hundred twelve) years ago, the first multicellular organisms appeared. They were low-organized and similar to