Sea urchins, although exhibiting radial symmetry, are part of the phylum Echinodermata, distinct from cnidarians like jellyfish and corals. The primary reason for this classification lies in their evolutionary traits and anatomical structure.
Cnidarians primarily have a simple body structure with two main layers of cells (ectoderm and endoderm) and exhibit specialized cells called cnidocytes, which are used for capturing prey. In contrast, sea urchins possess a more advanced body plan with a unique internal skeleton made of calcareous plates, known as ossicles, and they have a complex water vascular system that aids in movement and feeding.
Additionally, sea urchins reproduce differently than cnidarians, often through external fertilization in the water, while cnidarians may have more varied reproductive strategies, including asexual reproduction through budding.
In summary, while sea urchins and cnidarians share some superficial characteristics like radial symmetry, their deeper physiological and developmental differences are what ultimately place sea urchins firmly within the Echinodermata phylum.