Nov 1, 2014

Support and Movement in Animals

Animals are active organisms that require movement to obtain food and escape predators. Mobility is a characteristic of animals, and muscle contraction is necessary for movement. This muscle contraction must be directed against some sort of medium, such as internal body fluids, a rigid exoskeleton, or a rigid endoskeleton.

 

Hydrostatic Skeleton

Animals that lack a hard skeleton, a fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton. A hydrostatic skeleton offers support and resistance to the contraction of muscles so that mobility results. It is found in Cnidaria such as Hydra and flatworms such as Planaria, nematodes, annelids, and other soft-bodied invertebrates.

 

Examples:

Sea Anemone: The animal has a central digestive cavity filled with seawater and surrounded by the body wall. When the longitudinal muscles contract, the animal becomes shorter and wider, and when the circumferential muscles contract, the animals become longer and narrower.

Nematodes: The roundworms have a fluid-filled pseudocoelom and move in a whip-like manner when their longitudinal muscles contract.

Earthworm: Earthworms are segmented, and each segment has its own set of longitudinal and circular muscles. The hydrostatic skeleton consists of these fluid-filled segments separated by septa. By alternating circular muscle contraction and longitudinal muscle contraction, the animal moves forward.

 

Exoskeleton

An exoskeleton is an exterior skeleton to which internal muscles can be attached. The exoskeleton is inert and nonliving. Calcium carbonate forms the exoskeleton of some animals such as corals and of some mollusks e.g., marine bivalvia, snail.

 

Structure of Exoskeleton:

It is secreted by the ectoderm in animal cells. It is composed of two layers. The epicuticle is the outermost layer, which is made up of waxy lipoprotein. The bulk of exoskeleton below the epicuticle is called procuticle. The procuticle is composed of chitin, tough leathery polysaccharide, and several kinds of proteins. It is further hardened by deposition of CaCO3.

 

Examples:

Mollusks: The shell of mollusks consists of just one or two pieces. In some mollusks shell is composed of crystals of CaCO3 e.g. marine bivalvia, snail. The shell of land snail generally lacks the hard minerals and is lighter. The soft parts of the molluscan body have a hydrostatic skeleton as well.

Arthropods: They have exoskeleton but they have made adaptations, such as chitin, segmented exoskeleton, gaseous exchange, muscles, and moulting.

 

Endoskeleton

The skeleton found inside the body of vertebrates is called an endoskeleton. The only other animals with an internal skeleton are certain mollusks such as cuttlefish. In vertebrates, the skeleton is made of either cartilage or bone.

Evolution of Endoskeleton

The history of the development of the endoskeleton in various groups of vertebrates is known from the fossil record. The earliest vertebrates were fish. These fish had various adaptations for swimming. The backbone of fish is flexible, and generally, vertebrae are not interlocked for undulating movement.

 

Animals have different types of skeletons that allow them to move and adapt to their environments. The hydrostatic skeleton, exoskeleton, and endoskeleton all have unique features that help animals with movement and survival. The evolution of these skeletons has led to the diverse range of animals we see today.

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