Why are echinoderms grouped with the starfish and sea urchins?

Echinoderms are grouped with starfish and sea urchins because they share several distinctive characteristics that set them apart from other animal phyla. These shared features include:

1. Radial Symmetry: Echinoderms exhibit radial symmetry, meaning their body parts are arranged around a central axis. This symmetry is evident in the starfish's arms radiating from a central disc and the sea urchin's spherical body with spines extending outward.

2. Water Vascular System: Echinoderms possess a unique water vascular system that serves various functions, including locomotion, feeding, and respiration. This system consists of a network of canals and tube feet operated by hydraulic pressure. Starfish use their tube feet to move and capture prey, while sea urchins employ them for feeding and locomotion.

3. Endoskeleton: Echinoderms have an internal skeleton made up of calcareous plates or ossicles. These plates provide support and protection for their bodies. The intricate patterns and structures of these plates contribute to the distinctive appearance of different echinoderm species.

4. Tube Feet: As mentioned earlier, echinoderms have tube feet, which are hydraulically-powered structures used for locomotion, capturing prey, and sensing the environment. These tube feet are extensions of the water vascular system and are unique to echinoderms.

5. Coelom: Echinoderms possess a coelom, a fluid-filled body cavity that separates their internal organs from the body wall. This coelom provides space for the organs to move and function effectively.

6. Regeneration: Many echinoderms have remarkable regenerative abilities. They can regenerate lost arms or other body parts, ensuring their survival in challenging environments.

Based on these shared characteristics, echinoderms are grouped together as a phylum that includes starfish, sea urchins, sea cucumbers, brittle stars, and crinoids. Their unique radial symmetry, water vascular system, endoskeleton, tube feet, coelom, and regenerative capabilities distinguish them from other animal phyla.