In echinoderms skeleton is

Options

A. calcareous
B. chitinous
C. siliceous
D. tunicin made

Answer

### Step-by-Step Solution:

1. **Understanding Echinoderms**: Echinoderms are a group of marine animals that include starfish, sea urchins, and sea cucumbers. They are characterized by their radial symmetry and a unique water vascular system.

2. **Skeleton Composition**: The skeleton of echinoderms is primarily made up of calcium carbonate. This means that the skeleton is classified as calcareous.

3. **Types of Skeleton**: Echinoderms possess an endoskeleton, which is located inside the body rather than outside. This endoskeleton is composed of calcareous plates that provide structural support.

4. **Structure of the Skeleton**: The calcareous skeleton consists of various plates that can be tightly or loosely bound together. These plates are often covered by a layer of tissue called the epidermis.

5. **Crystal Formation**: Each skeletal element in echinoderms is typically a single crystal of calcium carbonate, and the arrangement of these crystals can be interlocked, contributing to the strength and rigidity of the skeleton.

6. **Eliminating Other Options**:
- **Chitinous**: This type of skeleton is found in arthropods (like insects and crustaceans) and is made of chitin, not calcium carbonate.
- **Siliceous**: This refers to skeletons made of silica, which is found in organisms like diatoms.
- **Tunicine**: This is a cellulose-like substance found in urochordates, which is unrelated to echinoderms.

7. **Conclusion**: Based on the characteristics and composition of the skeleton in echinoderms, the correct answer is that their skeleton is calcareous.

### Final Answer:
The skeleton in echinoderms is **calcareous**.

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