What are the Three Types of Skeletons?

The three types of skeleton are hydrostatic skeleton, endoskeleton and exoskeleton. Hydrostatic skeleton is found in cold-blooded animals including invertebrates. We human beings have endoskeleton. Exoskeleton is found in insects. We shall see in detail the three types of skeletons.

Hydrostatic Skeleton

It is found in soft-bodied and cold-blooded animals. This skeleton has a coelom, which is a fluid-filled cavity. This coelom is surrounded by muscles and the rigidity caused by the fluid and the muscles serve as a supporting structure for the organisms. The fluid pressure along with the motion of the supporting muscles helps the organisms to change shape and move. Invertebrates, the majority of the earth’s living organisms are found in a diverse number of habitats. They could be found in the deepest part of the oceans to the thickest jungles. These invertebrates have a hydrostatic skeleton system that helps them to thrive in a varied number of landscapes.

Echinoderms, cnidarians, annelids, nematodes and some other organisms use the hydrostatic skeleton for movement. The Earthworm which is an annelid is boneless. With the help of hydrostatic skeleton it burrows through the ground. Examples of echinoderms are the star fish and the sea urchin. The Jelly fish is a cnidarians.

Endoskeleton

The simplest definition for endoskeleton is that it is the skeleton found inside the body. It forms the frame work for the animal. The tissues and muscles are formed around the skeletal system and the muscular forces are transmitted to this skeleton. The Endoskeleton supports the animal structure. It is composed of mineralized tissues. In Phylum Chordata, Porifera and Echinodermata endoskeleton is present. Endoskeleton is found in the sub-class Coleoidae. The animals that come under Phylum Chordata are all vertebrates including human beings. Phylum Porifera are sponge like animals and has almost 5000 species. Its skeleton is a compound of organic fibres, a base of calcite and aragonite and spicules of silica. Here the endoskeleton is for support. Phylum Echinodermata consists of the various symmetrical marine animals like the star fish, sea urchins etc. It has an endoskeleton made of calcium and is covered with spines.

You might wonder whether Echinoderms have hydrostatic skeleton or endoskeleton. Echinoderms are endoskeletal since they have an internal calcareous skeleton. But for motion, it uses the tentacles which are similar to a hydrostatic skeleton.

The endoskeleton present in chordates and echinoderms are formed from mesodermal tissue and it is considered the real endoskeleton. In Coleoidae, the exoskeleton has evolved into the internal structure. Example is cuttle fish.

Exoskeleton

These are skeletons found outside the body. It forms a protective covering for the animals. It supports as well as protects the animals. All crustaceans have exoskeleton. Crabs, spiders, lobsters, insects are all crustaceans. Animals with exoskeleton are usually small. This is because large animals could not be supported by exoskeleton and need bones to support them. Animals with exoskeleton have a head and abdomen and in some cases, a thorax. The exoskeleton is soft and thin at the joints where it has to bend. The large exoskeletons are called shells. Tortoise is one animal that has a shell and endoskeleton.