Class bivalvia is characterised vby

Options

A. Presence of tentacles around mouth
B. Absence of head
C. Coiled shell
D. Absence of gills.

Answer

### Step-by-Step Solution

1. **Understanding Class Bivalvia**:
- Bivalvia is a class within the phylum Mollusca. Members of this class are commonly known as bivalves, which include clams, oysters, and mussels.

2. **Key Characteristics of Bivalvia**:
- Bivalves are characterized by having two shells (or valves) that are hinged together. These shells are made up of a right and left half.
- Unlike other mollusks, bivalves do not have a distinct head. This is a significant characteristic that sets them apart from other classes within the phylum.

3. **Absence of Head**:
- One of the defining features of bivalves is the absence of a well-defined head. They lack a proper cranium and do not possess tentacles around their mouth.
- Additionally, bivalves do not have a radula (a tongue-like organ with teeth) or jaws, which are common in other mollusks.

4. **Other Options Analysis**:
- The presence of tentacles around the mouth is not a characteristic of bivalves; this is more typical of cephalopods (another class of mollusks).
- The shell of bivalves is not coiled; this feature is seen in gastropods (another class of mollusks).
- Bivalves do possess gills, which are used for respiration and feeding, so the statement about the absence of gills is incorrect.

5. **Conclusion**:
- Based on the analysis, the correct characteristic that defines Class Bivalvia is the **absence of a head**.

### Final Answer:
Class Bivalvia is characterized by the **absence of head**.