Skeleton

Skeleton, the supportive framework of an animal body. The skeleton of invertebrates, which may be either external or internal, is composed of a variety of hard nonbony substances. The more complex skeletal system of vertebrates is made up of connective tissues. This includes bone and the several fibrous substances that form the joints that help to connect bone to bone and to muscle, it also encloses muscle bundles, and provides support by attaching the internal organs. For a more detailed discussion about the skeleton, go through the article.

Animal Skeleton

In addition to its supportive function, the animal skeletal system may provide protection, facilitate movement, and aid in certain sensory functions. A simple stiff, translucent, and nonliving envelope called a pellicle provides support to the body in many protozoans. In non-moving (sessile) coelenterates, such as coral, whose colonies attain great size, it is achieved by dead structures, both internal and external, which form supporting axes. 

In the many groups of animals that are able to move, the movement is achieved either by external structures that are called exoskeletons or by the internal structures that are called endoskeletons. Many of the animals remain in their normal resting positions or erect by means of a specialized skeleton called a hydrostatic skeleton.

The skeleton’s protective function is provided by the structures that are situated on the body surface, for example, the shells present on the crabs. These structures carry no muscle and form part of a protective surface armor. The similar structures that help in protecting are scales of fish, spines of echinoderms, the minute needle-like structures of sponges, and the tubes of hydroids. The bones present in the skull of the vertebrate helps to protect the brain. In most vertebrates and invertebrates, many skeletal structures provide protection and a rigid base for the insertion of muscles.

Let us discuss about the skeletal system types.

Types of Skeleton

There are certain types of skeletons that can characterize particular animal phyla, but in an animal, there are a limited number of ways in which it can form its skeleton. The calcareous internal skeleton present in the echinoderms is simply constructed but is not far different from the more elaborate bones of vertebrates. Skeletal fibers of similar chemical composition occur in unrelated animal groups; for example, coiled shells of roughly similar chemical composition are present in gastropods (e.g., snails), brachiopods (e.g., lamp shells), and cephalopods (e.g., chambered nautilus). The mechanical properties of various skeletal types may vary according to the needs of animals of particular habits or size ranges (e.g., aquatic, terrestrial).

The Different Types of Skeletal Systems are: 

1. Hard structures

2. Semi Rigid structures

3. Connective tissue

4. Hydrostatic structures

5. Elastic structures

6. Buoyancy devices

Calcareous Skeleton

The external shells found in the gastropods and bivalve mollusks are calcareous, stiff, and can be detached from the body. The laminated, or layered, shell grows by marginal and surface additions on the inner side. Muscles are present on the part of the shell, and the body of the animal that helps in the protection of the shell. The cephalopods and protozoans of the order Foraminifera form chambered calcareous shells that are numerous and large where the broken remains of the shells may form into a type of sand that covers large areas of tropical beaches, and the pieces may also convert into the rock. Protozoans of the order Radiolaria from skeletons of silica in the form of very complicated bars. 

Coral skeletons are also partly inside and partly outside the animal. Calcareous depositions below a young coral polyp are secreted by the ectoderm, fixed to the surface to which the animal is attached, and thrown up into ridges, which form a cup into which the polyp can contract. Most of the soft tissue is then external to an axial calcareous skeleton, but in rapidly growing corals the skeleton is perforated, and soft tissue lies both inside and outside it. 

The starfishes, brittlestars, and crinoids have various types of calcareous ossicles that are present in the mesoderm. These form units that have jointed segments along both the arms, spines that project from the body covering and articulate with ossicles, and calcareous jaws. Less well-organized calcareous deposits stiffen the body wall between the arms of the starfish.