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