The animal kingdom is
characterized by two main designs of the nervous system - diffused nervous
system and centralized nervous system.
Diffused Nervous System
The diffused nervous
system is found in cnidarians (Coelenterates) such as Hydra, Jellyfish, etc.
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| Nervous System Of Hydra |
The
nervous system of Hydra consists of a network of threads
that extends throughout the animal, made up of neurons with synapses between
them. Communication can occur from any point in the animal to any other point,
and there is no definite pattern that the message will follow. Hydra has no specialized
muscle cells, but most of the cells of the inner and outer cell layers of the
animal can contract with a strong stimulus, causing the whole organism to
react. There are also special receptor cells for any chemical or mechanical
change in the environment.
The centralized nervous
system is found in the animal kingdom from phylum platyhelminthes to chordates,
including humans. In this system, there is an increased number of nerve cells,
a concentration of nerve cells forming ganglia and a brain, a specialization of
function, an increased number of association neurons and synaptic complex, and
the formation of a head.
Planaria
has a centralized nervous system with a concentration
of neurons in the head region, which may be called a brain or cerebral
ganglion. From the brain, two longitudinal nerves emerge and extend up to the
pointed end of the body. From each longitudinal nerve, a series of lateral
nerves are given, consisting of neurons. There are also two eye spots that are
connected to the brain and have receptor cells sensitive to light, pressure,
touch, and chemical stimuli. The central nervous system, including the brain
and longitudinal nerves, contains neurons that transfer impulses from the
receptor cells to the motor neurons, which lead to the muscles. Removal of the
brain reduces the ability of movement in Planaria, indicating that the muscular
movement of Planaria is controlled by the brain.
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| Nervous System Of Planaria |
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