Final answer:
Echinoderms are invertebrate deuterostomes with juvenile bilateral symmetry and adult radial symmetry. They possess unique features like an internal skeleton of ossicles and a water-vascular system, and they share a closer genetic affinity to chordates than to other invertebrates.
Step-by-step explanation:
Invertebrate deuterostomes that are bilaterally symmetrical as juveniles but have radial symmetry as adults are called echinoderms.
Echinoderms, such as sea stars, sand dollars, and sea urchins, fall under the phylum Echinodermata. They exhibit a fascinating life cycle where they transition from bilateral symmetry in their larval stages to radial symmetry upon reaching adulthood. This feature is termed secondary radial symmetry, indicating that these organisms evolved from bilaterally symmetrical ancestors. Echinoderms are marine organisms characterized by an internal skeleton made up of calcareous ossicles, also known as body plates. Attributes such as epidermal spines attached to these ossicles, and a water-vascular system that assists in both respiration and locomotion, are unique to echinoderms. Additionally, echinoderms demonstrate a close genetic relationship to chordates, which includes animals with vertebral spines, despite their external differences. This connection enhances our understanding of the evolutionary history and developmental patterns within the animal kingdom.