Final answer:
Echinoderms and chordates are part of a monophyletic clade known as Deuterostomia, distinguished by their unique pattern of embryonic development where the mouth forms second.
Step-by-step explanation:
Based on ample evidence, the echinoderms and chordates form a monophyletic clade called the Deuterostomia. These two groups of organisms are unique in their development, as the mouth forms second during embryogenesis, hence the term deuterostome, which is derived from the Greek word meaning "mouth second". This is in contrast to protostomes, where the mouth develops first.
The Deuterostomia clade consists of two major phyla: Echinodermata and Chordata. Echinoderms include marine animals such as sea stars, sea urchins, and sea cucumbers, which all exhibit pentaradial symmetry and a spiny body covering. Chordates include vertebrates like fish, amphibians, reptiles, birds, and mammals, as well as two invertebrate subphyla, the urochordates and cephalochordates. These groups share features such as a tripartite coelom, pharyngeal slits, and a pattern of early development that is distinct from protostome animals.