Nervous System in Invertebrates:
(1) Nervous System in Cnidaria (Hydra, Sea-anemone)- The lowest animals with some sort of nervous system are Hydra and Sea-anemones (the coelenterates or Cnidarians). Their simple nervous system extends throughout the body within mesogloea in the form of a net of fine threads. The nerve net is formed of neurons that are connected with one another and with the contractile fibres of the epidermis and endodermis. The brain or something like the central nervous system is absent in this group.
(2) Nervous System in Flatworms- The brain and central nervous system appears for the first time in flatworms.
Planarians have a ladder-like nervous system. Their so-called brain is a concentration of neurons in the so-called head region. Two longitudinal nerve cords arise from the brain and extend on either lateral side up to the posterior end. Transverse nerves between the two nerve cords coordinate the movement of two sides.
(3) Nervous System in Annelids and Arthropods- Annelids (earthworm) and arthropods (prawns, crabs and insects) possess a typical invertebrate nervous system. These have-
- Central Nervous System (CNS) consisiting of brain and a solid ventral nerve cord which is segmentally ganglionated.
- Peripheral Nervous Syetm (PNS) consiting of nerves whic have both sensory and motor fibres.
The brain receives sensory information from the sense organs of the head and controls the activity of the ganglia. The nerves arising from the segmental ganglia of the ventral nerve control the action of segmental muscles of the body and body wall.