The correct option is A nerve net.
Explanation
Hydra belongs to the Phylum Cnidaria (Coelenterata), which exhibits a tissue level of organization. The neural system in these organisms is primitive and lacks the centralization found in higher animal phyla. It represents an early stage in the evolution of the nervous system.
Detailed Analysis:
- A nerve net is Correct: The neural system of Hydra is organized as a nerve net. This network consists of apolar neurons (neurons without distinct axons and dendrites) distributed throughout the body. This diffuse arrangement allows for the transmission of impulses in all directions, facilitating simple, coordinated movements of the body wall and tentacles.
- Brain with ganglia is Incorrect: A brain with ganglia represents a more advanced neural organization found in higher invertebrates, such as Arthropods (e.g., insects) and Annelids. Hydra lacks distinct ganglia or a centralized brain.
- Nerve cord is Incorrect: A nerve cord is a feature of bilaterally symmetrical animals. Non-chordates (like earthworms) typically have a ventral solid nerve cord, while Chordates possess a dorsal hollow nerve cord. Hydra does not possess a nerve cord.
- Cerebral cortex is Incorrect: The cerebral cortex is a highly evolved part of the vertebrate brain, particularly prominent in mammals. It is responsible for complex processing and is entirely absent in primitive invertebrates like Hydra.
Key Takeaway:
Cnidarians, such as Hydra, possess a primitive nervous system consisting of a nerve net made of apolar neurons. This system allows for non-directional impulse transmission and lacks a central control organ like a brain.