Identify the phylum in which endocrine system appeared for the first time.

Options

A. Arthropoda
B. Annelida
C. Mollusca
D. Protochordata

Answer

To identify the phylum in which the endocrine system appeared for the first time, follow these steps:

### Step-by-Step Solution:

1. **Understand the Endocrine System**: The endocrine system is a network of glands that produce and release hormones to regulate various bodily functions. It plays a crucial role in communication within the body.

2. **Identify the Phyla**: The question asks for the specific phylum where the endocrine system first appeared. We need to consider various animal phyla and their characteristics regarding the endocrine system.

3. **Research Phyla**:
- **Annelida**: This phylum includes segmented worms. The endocrine system is absent in this group.
- **Mollusca**: This phylum includes snails, clams, and squids. The endocrine system is present but developed after Arthropoda.
- **Arthropoda**: This phylum includes insects, arachnids, and crustaceans. It is known that the endocrine system was first developed in this group.

4. **Conclusion**: Based on the information gathered, the first appearance of the endocrine system is in the phylum **Arthropoda**.

### Final Answer:
The phylum in which the endocrine system appeared for the first time is **Arthropoda**.

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