Question

Describe nervous system in planaria with well labelled diagram.

Answer 1

Ladder type nervous system in Flatworms e.g. Planaria

1. Planaria is a flatworm belonging to the phylum platyhelminthes.
2. It is the most primitive animal with a central nervous system (CNS) located on the ventral side of the body.
3. It consists of a mass of cerebral or cephalic ganglions appearing like an inverted U-shaped brain.
4. These lie in the anterior or head region, and from each ganglion arise nine branches towards the outer side.
5. Ventrally from below the ganglia arise a pair of Ventral Nerve Cords (VNC), or long nerve cords.
6. These are interconnected to each other by a transfer nerve or commissure in a ladder-like manner.
7. The peripheral nerve plexus arises laterally from VNC.
8. The PNS include sensory cells arranged in lateral cords in the body. A pair of photosensory structures, the eyes are located on the dorsal side of the brain. Also, there are single sensory cells scattered in the body.
9. In the above examples of Hydra, Planaria, and the earlier studied examples of cockroaches and humans, we have seen the gradual evolution or changes of the neural system.
10. There is a high level of specialization in the formation of neurones as electrically signalling cells and also in the entire system, gradually from a diffuse neural system to a centralized nervous system.
11. The expansion into a properly organized system involving the brain, its gradual expansion in size and functions.
12. This has lead to centralization of various sense organs assisting in coordinating the internal environment with that of the external environment. Also, there is the evolution of a complex networking system that efficiently transmits signals between one part or organ of the body and another.