Solution

All echinoderms are marine and nearly all are benthic. The characteristics of adult echinoderms are the possession of water vascular system with external tube feet and a calcerous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. The larvae of echinoderms have bilateral symmetry but this is lost during metamorphosis, when their bodies are reorganised and develop the characteristic radial symmetry in adults. Hence, option D is correct.