Final answer:
Echinoderms and chordates form a monophyletic group called Deuterostomia defined by their unique embryonal development pattern, where the anus develops before the mouth, and share distinct characteristics such as a tripartite body plan, setting them apart from protostomes.
Step-by-step explanation:
The monophyletic clade that includes echinoderms and chordates is known as the Deuterostomia. Within this group, echinoderms are represented by organisms such as sea stars, sea urchins, and sand dollars, whereas chordates include a wide range of animals from vertebrates like humans to invertebrates such as lancelets. The classification as deuterostomes is pivotal because, unlike protostomes, the echinoderms and chordates exhibit an embryonic development where the mouth appears second, the anus develops first, and they possess other unique characteristics such as a tripartite body plan and a differentiated nervous system.
It is critical to understand the significance of this classification because it reveals a closer evolutionary relationship between echinoderms and chordates. This relationship is not initially apparent since echinoderms appear quite distinct in their physical morphology compared to chordates. However, the pattern of their embryonic development and genetic evidence strongly support this close phylogenetic kinship, differentiating them from protostomes like mollusks, annelids, and arthropods, which follow a different embryonic developmental sequence with the mouth developing before the anus.