Notochord occurs in the adult animal of

Options

A. Osteichthyes
B. Chondrichthyes
C. Protochordata
D. Amphibia

Answer

To solve the question "Notochord occurs in the adult animal of," we will analyze each of the provided options step by step.

### Step 1: Understanding the Notochord
The notochord is a rod-like structure that serves as a primary support in the early stages of development in chordates. It is a defining feature of the phylum Chordata.

**Hint:** Remember that the notochord is primarily present during the embryonic stages in many chordates.

### Step 2: Analyzing the Options
1. **Osteichthyes (Bony Fishes)**: In these animals, the notochord is replaced by a vertebral column as they mature. Therefore, they do not have a notochord in the adult stage.

**Hint:** Consider the structural changes that occur during the development of higher chordates.

2. **Chondrichthyes (Cartilaginous Fishes)**: Similar to osteichthyes, these fishes also lose their notochord and develop a vertebral column in adulthood.

**Hint:** Think about the common characteristics of vertebrates and how they differ from lower chordates.

3. **Protochordata**: This group includes lower chordates such as urochordates and cephalochordates. They retain the notochord throughout their adult life.

**Hint:** Identify which group of chordates retains the notochord into adulthood.

4. **Amphibia**: Amphibians, like frogs and salamanders, also belong to the higher chordates. They have a vertebral column in their adult form, and the notochord is present only in the embryonic stage.

**Hint:** Recall the classification of amphibians and their developmental characteristics.

### Step 3: Conclusion
Based on the analysis, the only group that retains the notochord in the adult stage is the **protochordates**. Therefore, the correct answer is **protochordata**.

**Final Answer:** The notochord occurs in the adult animal of **protochordata**.