Which of the following animals has a nervous system but no brain

Options

A. Sycon
B. Pleurobrachia
C. Hydra
D. Periplaneta

Answer

To solve the question "Which of the following animals has a nervous system but no brain?" we will analyze the options provided and determine which animal fits the criteria of having a nervous system without a centralized brain.

### Step-by-Step Solution:

1. **Understanding the Options**: The options given are:
- A) Psychon
- B) Pleurobrachia
- C) Hydra
- D) Periplaneta

2. **Defining Nervous System and Brain**:
- A nervous system is a network of neurons that allows for communication and coordination within an organism.
- A brain is a centralized organ that processes information and coordinates responses.

3. **Analyzing Each Option**:
- **Psychon**: This term is not commonly recognized in biological taxonomy. It may refer to a theoretical concept rather than a specific animal.
- **Pleurobrachia**: This is a genus of comb jellies (ctenophores) that possess a nerve net but also have a simple nerve structure that can be considered a primitive form of a brain.
- **Hydra**: Hydra is a simple organism belonging to the phylum Cnidaria. It has a decentralized nervous system known as a nerve net, which lacks a centralized brain. The nerve net allows for basic responses to stimuli.
- **Periplaneta**: This refers to the American cockroach, which has a well-developed nervous system including a brain and ganglia.

4. **Conclusion**: Among the options, Hydra is the only organism that has a nervous system but does not possess a brain. It has a nerve net that allows it to respond to environmental stimuli without a centralized processing unit.

### Final Answer:
The correct answer is **C) Hydra**.