Which of the following phyla is a small group of bilaterally symmetrical, triploblastic, and coelomate worm-like marine animals?

Phylum Hemichordata consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. Hemichordata is considered a minor phylum since it includes only a few species. 

Arthropoda is the largest phylum. 

Mollusca is the second largest phylum.

Adult echinoderms are radially symmetrical.