Practice question

Sea urchins are familiar marine invertebrates characterized by pentaradial symmetry as adults, water vascular system with hydraulic tube feet, endoskeleton composed of interlocking calcareous ossicles bearing spines, and classic deuterostome development where blastopore becomes anus and coelom forms enterocoelically. These anatomical and embryological traits uniquely define Phylum Echinodermata, class Echinoidea, alongside starfish, brittle stars and sea cucumbers. Their phylogenetic proximity to chordates makes them powerful model for gene regulatory network evolution. Mollusca, Arthropoda and Chordata lack this combination of radial adult symmetry, ambulacral system and mesodermal calcified skeleton, confirming echinoderm affinity.