Although no real nervous system is present in protozoa and sponges, coordination and reaction to external and internal stimuli such as, to light, temperature, chemicals etc do occur Only animals with tissue level of organization have true nervous system.
In diploblastic and triploblastic invertebrates (other than protozoa and sponges), five general evolutionary trends in nervous system development are seen
- First there is a nerve net with a bidirectional nerve impulse flow generally found in simple radially symmetrical animals, such as cnidarians or a slightly complex nerve net with some degree of “central” coordination (sea star) as in echinoderms, conduct nerve impulses from one area to other.
- Secondly, in animals that move in a forward direction (so they must have sense organs concentrated in the body region that first encounters new environmental stimuli), there is tendency towards centralization i.e., concentration of receptors and nervous tissue in the animal’s anterior end, as ganglia “brain” in flatworms and roundworms.
3 -The third trend in the evolution of nervous system in invertebrates coincides with the evolution of bilateral symmetry (in body plan with roughly equivalent right and left halves). There is development of paired neurons, paired sensory structure, muscles, and brain centres, which facilitates coordinated ambulatory movements, such as climbing, crawling, flying, or walking.
- The fourth evolutionary trend is seen in invertebrates, such as crustaceans, segmented worms (annelids), and arthropods in which axons join into nerve cords, and in addition to a small, centralized brain, smaller peripheral ganglia help coordinate outlying regions of the animal’s body. The more complex an animal, the more interneurons it has. Interneurons in ganglia integrate impulses (to effectors), so the more interneurons in an animal, more are the behaviour patterns.
- The fifth trend is a consequence of the increasing number of interneurons. The brain contains the largest number of neurons, and the more complex the animal, and the more complicated its behaviour, the more neurons are concentrated in the brain and the bilaterally organized ganglia. Vertebrate brains are an excellent example of this trend. Fig. P.9
A brief description of some invertebrate nervous systems is illustrated in the diagram 2.9.