Protozoa
§ Protozoans lack specialized nervous system to respond to the environment.
§ Single cell function as both the receptor and the effector.
· Contain an eyespot that act as a light sensitive receptor.
Porifera
§ Poriferans or the sponges are the only multicellular animals without a nervous system. They do not show any neurons or sensory cells.
§ Although they lack a nervous system they are sensitive to pressure and touch that helps in their locomotion.
Cnidaria
§ Contains a diffuse nerve net.
§ Sensory neurons, intermediate neurons and mortor neurons are present
§ These neurons are connected together by synapses.
§ Synapses carry impulses to both directions within the nerve net.
§ In every syanpse, the impulse ends.
§ Impulse is used to charge the synapse. This is called as ‘Facilitation’.
§ This lets the other on coming impulse to go through the syanpse. This process is known as ‘Decremental conduction’
§ Therefore impulses traspitation is slow.
· Nerve net is present.
· Contain multipolar neurons.
-Nerve net of cnidarians-
Aurelia sp and Sea anemones.
· Nerve net is present.
· Also bipolar neurons run through conduction tracts.
· Therefore transpitation of impulses are much faster.
· Some have specialized stuctures called ‘Rhopalia’ which have receptors for light, balance, touch and chemical detection.
§ Ocelli: For light
§ Statocysts: For balance
§ Sensory lappets: For touch
Platyhelminthes
§ Bilateral symmetry and cephalization have led to the nervous system.
§ Shows the primitive arrangement of the central nervous system.
§ Nervous system resembles a ladder.
§ Two long nerves are connected to the cerebral ganglia located in the head region.
§ Short, smaller nerves are connected to the nerve cord.
§ Auricles can be found at the sides of the head.
§ These contain sensory receptors.
§ Eyespots are also present.
§ At the anterior end cerebral ganglion is present composed of dense circular nerve ring surrounding the pharynx.
§ Arising from the nerve ring four nerves run along the length of the body as one dorsal, one ventral and two lateral nerves.
§ Each nerve is located within a cord of connective tissue lying beneath the cuticle and between muscle cells.
§ Dorsal nerve trunk is motor, lateral trunks are sensory and ventral nerve function as both.
Below diagram shows the location of the nerve cord in relation to the muscle cells in nematode’s body.
Annelida
§ Nervous system is segmented.
§ Pair of cerebral ganglia (supra-pharyngeal) situated above and in front the pharynx.
§ Two lateral nerves (circum or pharyngeal connectives) form a ring around the pharynx connecting the ganglia.
§ A pair of mid ventrally placed nerves that connected to the anterior nerve ring, run throughout the length of the body. These nerves have a ganglion in each of the body segment from which nerves are given out to various organs.
§ Most annelids have giant axons.
§ Some annelids have ocelli or simple eyes.
-Nervous system of Pheretima sp-
Mollusca
§ Shows great range of nervous systems.
§ Typical nervous system of Mollusks is composed of three pairs of ganglia connected with one another by bundles of nerve fibers but distributed in a characteristically scattered manner.
§ One pair of ganglia above the oesophagus is called as "supra-oesophageal" or "cerebral" ganglion.
§ A pair of ganglia below the oesophagus is called as "infra-oesophageal" or "pedal" ganglion.
§ The other pair of ganglia is called as "branchial" or "parieto-splanchnic" ganglion.
§ In higher Molluscs, the cerebral and pedal ganglia are fused forming an oesophageal ring.
-Typical nervous system of a mollusk. n- Cerebral ganglion, n'- Pedal ganglion, n"- Parieto-splanchnic ganglion.-
Aplysia sp
· Several ganglia are present that are connected with long nerves.
Arthropoda
§ Similar to annelid nervous system but more advanced in structure.
§ Central nervous system is present.
§ Supra-oesophageal or cerebral ganglion is located in the head segment. This serves as the brain.
§ Sub-esophageal lies immediately behind the brain.
§ It’s composed of three fused ganglia.
§ A pair of dense ventral nerve cords that are linked with the above ganglia run along the length of the body.
§ In each body segment these nerve cords contain a ganglion which gives out nerves to the other internal organs.
§ Most arthropods show well developed sensory organs such as compound eyes and antennae.
§ Compound eyes contain ‘Ommatidia’ that samples a small part of the visual field.
-Nervous system of an insect-
-Structure of ommatidia-
Echinodermata
§ Although echinoderms are deuterostomes, their central nervous system is different from the other deuterostomes.
§ Radial nerves run under each of the ambulacra. Some echinoderms show ganglia along the radial nerves.
§ They include the cell bodies of almost all motor neurons and intermediate neurons.
§ A central nerve ring surrounds the gut connecting the radial nerves.
§ Some shows Ocelli to sense light.
§ Sensory neurons lie within the ectoderm of podia, and send axons to the radial nerves.
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