In echinoderms, ossicles are

Options

A. Exoskeletal plates
B. Endoskeletal plates
C. Regions of ambulacral grooves
D. Regions from where oral tentacles develop.

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Question:**
The question asks about the nature of ossicles in echinoderms. We need to identify what ossicles are among the given options.

2. **Analyzing the Options:**
- **Option A: Exoskeletal plates**
- Echinoderms do not have exoskeletal plates; they possess endoskeletal structures. Therefore, this option is incorrect.

- **Option B: Endoskeletal plates**
- Echinoderms have mesodermal endoskeletal plates that are calcareous. These plates are specifically referred to as ossicles. This option is correct.

- **Option C: Regions of ambulacral grooves**
- While echinoderms do have ambulacral grooves, ossicles are not limited to these regions. They are present uniformly over the body surface. Thus, this option is incorrect.

- **Option D: Regions from where oral tentacles develop**
- Oral tentacles are found around the mouth of echinoderms, whereas ossicles are distributed over the surface. Therefore, this option is also incorrect.

3. **Conclusion:**
The correct answer is **Option B: Endoskeletal plates**. In echinoderms, ossicles are mesodermal endoskeletal plates that are calcareous.

---