What is the basis of classification of Protochordata?

Options

A. Gut
B. Brain
C. Gills
D. Notochord

Answer

### Step-by-Step Solution

1. **Definition of Protochordata**: Protochordata, also known as Hemichordata, is a group of animals that serve as an evolutionary link between invertebrates and vertebrates.

2. **Characteristics of Protochordata**:
- They possess both invertebrate and vertebrate characteristics.
- They lack a true nerve cord, which is a characteristic of invertebrates.
- They have a notochord, which is a characteristic of vertebrates.

3. **Body Organization**: The body of Protochordata is divided into three main parts:
- **Proboscis**: The anterior part of the body.
- **Collar**: The middle part of the body.
- **Trunk**: The posterior part of the body.

4. **Classification Basis**: The classification of Protochordata is based on several key features:
- **Gut Development**: The gut system is generally undeveloped.
- **Brain Development**: The brain is poorly developed or may even be absent.
- **Gills**: Gills are absent in Protochordata.
- **Notochord**: The presence of a notochord, which is typically found only in the upper part of the body, is a significant characteristic for classification.

5. **Conclusion**: The presence of the notochord is a defining feature that classifies Protochordata and distinguishes it from other groups. This characteristic supports the idea that Protochordata forms a transitional link between invertebrates and vertebrates.