Teeth are modified _________ scales which are backwardly directly

Options

A. Ctenoid
B. Cycloid
C. Placoid
D. Ganoid

Answer

To solve the question "Teeth are modified _________ scales which are backwardly directed," we need to identify the type of scales from which teeth are derived and understand their characteristics.

### Step-by-Step Solution:

1. **Understanding the Question**: The question is asking about the type of scales that are modified to form teeth, specifically noting that these scales are directed backward.

2. **Identifying Scale Types**: There are different types of scales in fishes:
- **Cycloid Scales**: These are smooth and circular.
- **Ctenoid Scales**: These have a toothed edge.
- **Ganoid Scales**: These are diamond-shaped and have a bony structure.
- **Placoid Scales**: These are small, tooth-like structures found in sharks and rays.

3. **Analyzing the Function of Teeth**: In sharks and rays, teeth are not just for eating but also serve a protective function. This indicates that the type of scale from which they are derived must be robust and effective.

4. **Connecting Scales to Teeth**: Among the types of scales listed, placoid scales are the ones that are modified into teeth. They are characterized by their backward direction and their sharp, pointed structure, which is similar to teeth.

5. **Final Answer**: Therefore, the correct completion of the sentence is: "Teeth are modified **placoid** scales which are backwardly directed."