Chondrichthyes can be differentiated from osteichthydes externally by

Options

A. Naked gills
B. Heterocercal tail
C. Ventral mouth
D. All the above

Answer

To differentiate Chondrichthyes from Osteichthyes externally, we can follow these steps:

### Step-by-Step Solution:

1. **Gills Structure**:
- Chondrichthyes (cartilaginous fish) have exposed gills that are not covered by an operculum (a bony flap). This means their gill slits are visible on the sides of their bodies.
- In contrast, Osteichthyes (bony fish) have gills that are covered by an operculum, which provides protection and aids in respiration.

2. **Mouth Position**:
- The mouth of Chondrichthyes is located on the ventral (underside) part of their body. This positioning is typical for many species within this group.
- Osteichthyes, on the other hand, generally have a terminal mouth, which is located at the front of their heads.

3. **Tail Structure**:
- Chondrichthyes possess a heterocercal tail, which means that the tail has two lobes that are asymmetrical in shape. This tail structure helps in providing lift and thrust.
- Osteichthyes typically have a homocercal tail, where the tail fin is symmetrical and consists of two lobes of equal size.

4. **Conclusion**:
- Based on these differences in gill structure, mouth position, and tail shape, we can conclude that Chondrichthyes can be differentiated from Osteichthyes externally by all of the mentioned characteristics.

### Final Answer:
All of the above (naked gills, heterocercal tail, ventral mouth).

---