# (i) 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.

# (ii) 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 addition to the centralized nervous system planaria also possesses a sub-epidermal nerve net like that of coelenterates. Brain receives stimuli from the sense organs and conveys them to different parts of body. Special receptors, as found in turbellarians, are lacking in tapeworm. However numerous free sensory nerve-endings are present throughout the body specially in the scolex.
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.

# (iii) 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.

# (iv) Arthopoda : The nervous system of prawn or arthopods 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.

# (v) 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, supraintestinal 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.’

# (vi) 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.’

# (vii)Hemichordata : Nervous system is of primitive type resembling that of coelenterates and echinodermates.
Chordates : Nervous system well developed and formed by ectoderm. It is formed by CNS, peripheral nervous system and autonomous N.S.

The central nervous system of vertebrates includes the brain and the spinal cord. These are derived from a longitudinal mid-dorsal ectodermal thickening of the embryo, called the meduallary or neural plate. This neural plate or neural groove is converted by fusion into a closed mid-dorsal longitudinal neural tube lying above the notochord. Histologically, the embryonic neural tube exhibits three zones of cells.
# (i) Germinal layer : These are actively dividing cells lining the neural canal. They form the connective tissue lining of neural canal, called ependyma, and also proliferate into mantle layer cells.

# (ii) Mantle layer : It consists of embryonic neurons or nematoblasts, forming the gray matter.

# (iii) Marginal layer : It consists of nerve fibres, mostly surrounded by fatty myelin sheaths, and forms the white matter. Neurons and fibres are supported by a special connective tissue of ectodermal origin, the neuroglia, cells of which become increasingly abundant and diversified in higher vertebrates.

- Development of brain : The anterior end of embryonic neural tube is already enlarged forming the embryonic brain, called encephalon. By differential growth and two constrictions, it is divided into a linear series of three primary cerebral vesicles, termed the forebrain, midbrain and hindbrain. These give rise to the three major divisions of the adult brain – (1) prosencephalon (forebrain), (2) mesencephalon (midbrain), and (3) rhombencephalon (hindbrain). These further become subdivided into 5 subdivisions. The various parts of the adult brain in different vertebrates are formed by modifications. That is, by thickenings and foldings of these 5 subdivisions. The adult brain has a series of cavities, called ventricles, which are in continuation with the central canal of the spinal cord and filled with a cerebro-spinal fluid.

Nerve cells (= neurons) : Irritability is a basic characteristic of the “living substance”, i.e., the protoplasm. Consequently, every living cell becomes excited when stimulated. However, the nerve cells and muscle fibres are specialized excitable cells of body, capable of transmitting or conducting excitations along their membranes. Of these, muscle cells are further specialized for contraction while nerve cells are further specialized for receiving stimuli (as sensory or receptor cells) and transferring excitations from one to the other.
A typical neuron consists of a nucleated cell body (= cyton, soma or perikaryon), five to seven short, slender and branched (= arborized) dendrites, and a single, relatively thicker and longer fibrous axon. The latter is terminally branched into short telodendria. Each telodendron bears a terminal knob or bouton. Boutons of one neuron lie upon dendrites or cytons of adjacent neurons (figure), or upon muscle fibres or glands.
Nerve fibres : Although, all parts of a neuron transmit excitations (= impulses), but the transmission is always unidirectional. The dendrites and cytons usually constitute the impulse receiving parts which receive impulses directly from receptors, or from other adjacent neurons. The axons are specialized as fibres conducting impulses away from the receiving parts. Thus, the reaction or response impulses are always carried to the effectors by axons. That is why, the term ‘nerve fibres’ is usually applied to the axons. The latter are 0.1 mm to one or more (upto 10) metres long and about 0.025 m thick on an average.
Main properties of nervous tissue : The nervous tissue has two outstanding properties excitability and conductivity.
(1) Excitability : It is the ability of the nerve cells and fibres to enter into an active state called the state of excitation in response to a stimulus. Excitation arises at the receptors on account of various stimuli such as light, temperature, chemical, electrical or pressure which constantly act on the organisms.
(2) Conductivity : The excitation does not remain at the site of its origin. It is transmitted along nerve fibres. The transmission of excitation in a particular direction is called conductivity.