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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