Which one of the following animals possesses nerve cells but no nerves? (a) Hydra (b) Tapeworm (c) Earthworm (d) Frog’s tadpole
Option A Hydra possesses a very primitive type of nervous system. It includes bipolar and multipolar nerve cells or neurones lying immediately above the muscle processes and forming an irregular and discontinuous nerve net or nerve plexus Neighbouring nerve cells are not fused together, but their processes or neurites form synaptic junctions. Such a nerve net is called a synaptic nerve net. Nerve cells are numerous around mouth and on pedal disc but show no groupings in the form of a nerve controlling centre like brain or nerve ring. A difference from higher animals is that the nerve net of Hydra is unpolarized so that impulses can pass in all directions (diffuse transmission). In brief, nerve net shows diffuse unpolarized transmission, autonomy of parts and paucity of reflexes.
