Original: Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates; they are also the largest phylum that has no freshwater or terrestrial representatives.
Simplified: Echinodermata is the largest animal phylum which has no freshwater or terrestrial representatives.