In which animal, nerve cell is present but brain is absent ?

Options

A. Sponge
B. Earthworm
C. Cockroach
D. Hydra

Answer

To determine which animal has nerve cells but lacks a brain, we can analyze each option provided:

1. **Sponge**: Sponges are simple organisms that do not have a nervous system at all. They lack nerve cells, so they cannot be the answer.

2. **Earthworm**: Earthworms possess a well-defined nervous system, including a brain-like structure and a ventral nerve cord with ganglia. Therefore, they cannot be the answer since they do have a brain.

3. **Cockroach**: Cockroaches also have a complex nervous system, including a brain and a ventral nerve cord with ganglia. Thus, they cannot be the answer either.

4. **Hydra**: Hydra is a simple organism that has nerve cells but does not have a centralized brain. It belongs to the phylum Cnidaria and has a decentralized nerve net instead of a brain.

Based on this analysis, the animal that has nerve cells but no brain is **Hydra**.

### Steps:
1. Analyze the characteristics of each animal listed in the options.
2. Identify the presence or absence of a nervous system and brain in each animal.
3. Conclude that Hydra is the only animal with nerve cells but without a brain.