Final answer:
Echinoderms are marine organisms found in various marine environments, such as shallow waters, deep seas, and especially in reefs. They have a unique water vascular system used for movement and respiration. All echinoderms live in saltwater, making them strictly sea dwellers.
Step-by-step explanation:
Echinoderms are typically marine dwellers and inhabit various types of oceanic waters. They are exclusively found in marine environments, ranging from shallow waters to the deep sea. Sea urchins and sand dollars are types of echinoderms from the Class Echinoidea, and they are known to live on the ocean floor. Sea urchins can be either herbivores, consuming kelp and algae, or carnivores, feeding on entities such as bryozoans. Furthermore, echinoderms like sea cucumbers are common in deep ocean habitats, sometimes comprising up to 90% of the organisms there.
The water vascular system, a distinctive feature of echinoderms, is essential for their respiration and locomotion. This system also plays a role in their ability to move and feed. The water vascular system is instrumental for sea urchins and their movement along the ocean floor, while their feeding structure, known as Aristotle's lantern, is involved in grazing on algae or suspension feeding.
While they have varied habitats and feeding habits, no echinoderms are found in freshwater or terrestrial environments, making them a fundamentally ocean-based group of species in the animal kingdom.