Platyhelminthes - Nervous Systems
Among Platyhelminthes, you will observe that turbellarians have a brain and three to five pairs of longitudinal nerve cords extending from the brain to the posterior end. They are positioned along dorsal, lateral and ventral aspects. The cords are connected together through means of commissures. The commissures are in turn connected to a sub-muscular nerve plexus.
Figure: Planarian flatworms have a ladder-type nervous system. Cerebral ganglia in the head region serve as a simple brain and, to some extent, control the rest of the nervous system.
So, the system is not only sunk deeper into the sub-muscular position, but it does not correspond to the real primitive plexus at all. The motor and association neurons are mainly confined to the brain and the cords all along their periphery surrounding the central neuropile. One pair of these longitudinal cords becomes conspicuous, the others tending to become weaker finally disappearing thus giving rise to the typical ladder such as nervous system. With the development of the central nervous system, movements become more co- ordinate. Parts of the animal can keep spontaneous movements and show co- ordination of responses in most species of this group, just only if it contains portion of the nerve cord. The brain appears to have started exerting a dominating affect in this group.