Unicellular organisms have a simple organisation of their life processes. However, a multicellular organisms body organs and organ systems requires a control over their life processes as well as a co-ordination between the various systems to maintain homeostasis of the organism.
Plants and animals both show a control and coordination mechanism.
In plants, control and co-ordination by sending chemical signals and brining about various types of movements takes place. (e.g. phototropic, chemotactic, thingmotactic (response to stimulus of touch or contact), etc.
In Animals, specially the higher vertebrates show a gradual increase in the complexity of their control and coordination by giving both electrical and chemical signals.
The porifera (sponges) are the most primitive of the animal phyla. Even though there are different types of cells in sponges for carrying out different functions, a proper nervous system is absent.
However, to bring about efficient working of the body these multicellular animals show division of labour among the cells. This leads to specialization of cells for the various activities like digestion, respiration, excretion and others.
Later in the higher animals, phyla the different cells forms tissues, organs and systems, which must work in coordination with each other for smooth internal functioning of the body. Also, the organism must be able to respond and coordinate with respect to various stimuli or changes in the external environment.
In the lower animals like Hydra and Planaria the control and co-ordination is carried out by nervous system, while in higher more complex animals, it is done by two coordinating systems - the nervous (electrical) system and endocrine (chemical) control system.
NERVOUS COORDINATION
Nervous System in Hydra:
v Hydra, a cnidarian shows primitive nervous system in the form of diffused nervous system.
v The cnidarians are the first animal group showing true simple nervous system.
v It consists of the sensory cells and the nerve cells or neurons along with their fibers.
v The nerve cells are distributed throughout the body and interconnected to each other by synapses between their fibers to form the nerve net.
v There are two nerve nets present in the mesoglea (hydrostatic skeleton)-
a.one connected towards the epidermis
b. second towards the gastro-dermis.
v There are sensory cells scattered in the body wall and tentacles, but sense organs are absent.
v Neurons have fibers but there are no sensory and motor nerves.
v The nerve impulse shows no polarity or direction.
v In Hydra, activation of sensory cells produced at any point, and from this point impulse can be carried throughout the body in any direction, thus bringing movements of the body or tentacles e.g. catching of prey during feeding.
v The diffused type of nervous system is the first important landmark in the nervous system.
v It is seen in the phylum Ctenophora as well as in the enteric system or gut wall of higher animals including man.
Nervous System in Planaria (flatworm):
1. Planaria is a flatworm belonging to the phylum Platyhelminthes.
2. It is the most primitive animal with a central nervous system (CNS) located on the ventral side of body.
3. It consists of a mass of cerebral or cephalic ganglion appearing like an inverted U-shaped brain.
4. These lie in the anterior or head region and from each ganglion arise nine branches towards the outer side.
5. Ventrally from cerebral ganglia arise a pair of Ventral Nerve Cords (VNC) which runs posteriorly, are inter connected to each other by transvers nerve or commissure in a ladder like manner.
6. From the ventral nerve cord arises peripheral nerve plexus.
7. The PNS include sensory cells arranged in lateral cords in the body.
8. A pair of photo sensory structure, the eyes are located on dorsal side of the brain and single sensory cells scattered in the body.
