When the word "skeleton" is heard, we usually immediately imagine a bare skull and spine, connected by many different bones. It really is, but not in all organisms on our planet. Many animals have an external skeleton. How it looks and what functions it performs, you will learn further.
What is the exoskeleton?
Muscles, ligaments and skeleton together form the musculoskeletal system of the body. Thanks to them, everything happens, even the smallest movements in terms of effort. The skeleton in this system plays a passive role. This is a frame that serves as a support for the muscles and protection for the internal organs.
He happens:
- internal;
- external;
- hydrostatic.
The least common hydrostatic skeleton. It is devoid of solid parts and is characteristic only of soft-bodied jellyfish, worms and sea anemones. All vertebrates have an internal, or endoskeleton. It consists of bones and cartilage, completely covered with body tissues.
The external skeleton is characteristic mainly of invertebrates, but may also be present invertebrates. It does not hide inside the body, but completely or partially covers it from above. The exoskeleton is made up of various organic and inorganic compounds, such as chitin, keratin, limestone, etc.
Not all organisms have only one type of "skeleton". Some species have both internal and external skeletons. Such animals include turtles and armadillos.
Polyps
Polyps are one of the most "lazy" creatures on the planet. They chose to practically not move on their own, but to live, clinging to the seabed, like plants. Only sea anemones do not have a hard skeleton. For the rest, it is represented by protein (gorgonians, black corals) or lime (madrepores).
The calcareous outer skeleton is commonly referred to as coral. In its small holes are the polyps themselves, connected to each other by a membrane of living tissues. Animals form whole numerous colonies. Together, their exoskeletons form an "underwater forest" or reefs that host entire islands.
The main part of the reefs is located in the waters of Southeast Asia. The largest colony in the world is the Great Barrier Reef in Australia. It stretches for 2500 kilometers and holds more than 900 islands.
Shellfish
Mollusks have one of the most beautiful and varied external skeletons. Science knows about two hundred thousand species of these animals, each of which has its own structure. The exoskeleton of most mollusks is represented by a shell. It may include aragonite or conchiolin with impurities of calcite, vaterite,calcium carbonate and calcium carbonate.
Some animals have a spiral shell, the curls of which twist in a circle (snails) or in the form of a cone (staircase epitonium). At the wide end there is a hole - the mouth. It can be narrow and wide, oval, round or in the form of a long slit.
In the cypree or knotweed, each new curl overlaps the previous one, which is why the spiral is poorly distinguishable, and it seems that it does not exist at all. But the bivalves really do not have it. Their shell consists of two convex symmetrical parts that open and close like a jewelry box.
Mollusc skeletons are usually not smooth. They are covered with microscopic scales, furrows and bulges. In some species, spines, keels, ridges and plates of variations of calcium carbonate extend from the shells.
Arthropods
The phylum Arthropod includes crustaceans, insects, arachnids, and centipedes. Their body has a clear shape and is divided into segments. In this regard, the external skeleton of arthropods is very different from the integuments of corals and mollusks.
Each segment of their body is enveloped by strong cuticles (sclerites) made of chitin and other impurities, which are interconnected by elastic and flexible membranes, providing mobility to the animal.
In insects, the strong but elastic cuticle represents the outer layer of the skeleton. Below it is a layer of hypodermis and basement membranes. It consists of fat-protein complexes that do not giveanimals to dry up.
In crustaceans, the cuticle is more durable and saturated with lime, which becomes more and more over time. In some species, the skeleton can be transparent and soft.
The cuticle contains pigments that give animals a variety of colors. From above, it is usually covered with scales, outgrowths and hairs (chaetoids). In some representatives, the integument is equipped with glands that secrete poison or odorous substances.
Vertebrates
Strong outer coverings are also found among more developed animals. The external skeleton of turtles is represented by a shell. It is a reliable protection for the animal, as it is able to withstand the weight of two hundred times the weight of its owner.
The shell consists of a thick upper keratin layer in the form of tightly fastened shields and an inner bone layer. From the inside, the spine and ribs are attached to them, repeating the arched shape of the shell. The part of the skeleton that covers the back is called the carapace, and the ventral shield is called the plastron. All scutes on them grow independently of the others and acquire annual rings when the animal falls into winter sleep.
The shells can have different colors and patterns, but basically their color is disguised as an external environment. Star turtles have black and bulbous scutes with yellow "stars" in the center. The African kynyx is more restrained and has a solid yellow-brown color.
