Teeth are modified _________ scales which are backwardly directly

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the type of scales that are modified into teeth and are directed backward.

2. **Identifying the Groups**: In the animal kingdom, particularly in the superclass Pisces (fishes), there are two main classes:
- **Osteichthyes**: Bony fishes with different types of scales.
- **Chondrichthyes**: Cartilaginous fishes, such as sharks and rays.

3. **Types of Scales**:
- In Osteichthyes, the scales can be of types like cycloid and ctenoid.
- In Chondrichthyes, the scales are known as placoid scales.

4. **Characteristics of Placoid Scales**:
- Placoid scales are unique to cartilaginous fishes and are also found in their teeth.
- These scales are characterized by their backward direction, which helps in preventing prey from escaping once captured.

5. **Conclusion**: Given that the question specifically mentions "teeth" that are modified scales and are directed backward, the correct answer is **placoid scales**.

**Final Answer**: Teeth are modified **placoid** scales which are backwardly directed.

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