Question

Describe nervous system in planaria with well labelled diagram.

Hint:

The key phrase for the planarian nervous system is "\textbf{ladder-like}." It represents a major evolutionary step: a centralized system with a primitive brain at one end for processing information.

Answer 1

The nervous system of a Planarian (a type of flatworm) is a primitive centralized nervous system that is more advanced than the diffuse nerve net found in cnidarians. It is often described as a ""ladder-like"" nervous system due to its structure. Structure:

Brain (Cerebral Ganglia): At the anterior (head) end of the planarian, there is a bilobed, mass of nerve cells called the cerebral ganglia. This structure functions as a simple brain, coordinating nerve signals and responses. 
Longitudinal Nerve Cords: Extending posteriorly from the brain are two main, parallel ventral longitudinal nerve cords that run the length of the body. 
Transverse Commissures: The two longitudinal nerve cords are connected to each other at regular intervals by a series of transverse nerve cords called commissures. This arrangement of longitudinal cords and transverse commissures gives the nervous system its characteristic ladder-like appearance. 
Peripheral Nerves: Numerous smaller peripheral nerves branch out from the longitudinal cords and the brain to innervate the muscles and sensory cells throughout the body, forming a peripheral nerve plexus. 
Sensory Organs: Planaria exhibit cephalization, with sensory organs concentrated at the anterior end. These include: 

Eyespots (Ocelli): Two cup-shaped eyespots are located on the dorsal side of the head. They are photoreceptive and can detect light intensity and direction, but they cannot form images. 
Auricles: These are chemosensory lobes on the sides of the head that help the planarian locate food. 

Diagram (Nervous System of Planaria):