Echinoderms are

Options

A. Exclusively fresh water forms
B. Exclusively marine
C. Estuarine
D. Estuarine and fresh water living

Answer

### Step-by-Step Solution:

1. **Definition of Echinoderms**: Echinoderms are a group of marine invertebrates that belong to the phylum Echinodermata. They are characterized by their unique body structure and systems.

2. **Symmetry**: Echinoderms exhibit bilateral symmetry in their larval forms, which means that the left and right sides of the larvae are mirror images of each other. However, as they mature into adults, they display radial symmetry, where their body parts are arranged around a central axis.

3. **Habitat**: All echinoderms are exclusively found in marine environments. This is due to their specialized adaptations that allow them to thrive in oceanic conditions.

4. **Water Vascular System**: One of the most distinctive features of echinoderms is their water vascular system. This system is unique to echinoderms and plays a crucial role in various functions such as locomotion (movement), respiration (breathing), and feeding.

5. **Conclusion**: Given these characteristics, echinoderms are a unique group of marine invertebrates that are defined by their radial symmetry in adulthood, bilateral symmetry in larval stages, and the presence of a water vascular system.