Category : 11th Class
(1) Coelenterata : True nerve cell or ganglion cells occur for the first time in coelenterates. They are derived from
interstitial cells of epidermis, forming nerve net or nerve plexus below whole epidermis. A polar neurons are found in coelenterata.
(2) Platyhelminthes : Nervous system of planarians marks the beginning of a centralized nervous system encountered in higher animals. That is made up of brain or cerebral ganglia, two lateral longitudinal nerve chords, numerous peripheral nerves and transverse commissures or connectives. This is sometimes called the ladder type of nervous system.
In Nematoda (e.g. ascaris) these system made up of central nervous system, peripheral nervous system and rectal nervous system. Rectal nervous system more developed in male. Ascaris with dorsal, ventral, and lateral nerve cords.
(3) Annelida : Nervous system well developed and concentrated. It consists of three parts : central nervous system, peripheral nervous system and sympathetic nervous system, central N.S. made up of Nerve ring and ventral nerve cord. Nerves are of mixed type, consisting of both afferent (sensory) and efferent (motor) fibres.
(4) Arthropoda : The nervous system of prawn or arthropods is of the annelidan type. However it is somewhat larger and has more fusion of ganglia. It consists of (i) The central nervous system including brain connected with a ventral ganglionated nerve cord through a pair of circum-oesophageal commissures, (ii) The peripheral nervous system including nerves and (iii) The sympathetic nervous system. In arthropods like cockroach sympathetic nervous system also known as stomatogastric nervous system, made up of 4 gonglion and retro-cerebral complex.
(5) Mollusca : In gastropodes (e.g. pila) consists of paired ganglia, commissures and connective uniting them and nerves running from these central organs to all parts of the body. It has various type of ganglia as cerebral, buccal, pleuro-pedal, supra intestinal and visceral etc. In palecypoda nervous system is greatly reduced due to sluggish and sedentary mode of life and there is little evidence of the brain. But in cephalopoda shows a high grade of organization attained only by some insects and arachnids among the other invertebrates.
(6) Echinodermata : Echinodermates has simple and primitive type nervous system. It has the form of a nerve net, consisting of nerve fibres and a few ganglion cells, all confined to the body wall except the visceral nerve plexus situated in the gut wall. At certain places the nervous tissue is concentrated to form distinct nerve cords. It is made up of (i) Superficial or ectoneural nervous system (ii) Hyponeural or deep nervous system (iii) Aboral or coelomic nervous system and (iv) Visceral nervous system.
(7) Hemichordata : Nervous system is of primitive type resembling that of coelenterates and echinodermates, with both dorsal and ventral nerve cord.
(8) Chordates : Nervous system well developed and formed by ectoderm. It is formed by central nervous system, peripheral nervous system and autonomic nervous system.
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