The lesser deuterostomates
- Arrowworms - marine; resemble arrow-shaped nematodes and annelids;
demonstrate deuterostomate embryo development; feed on plankton and small
fish (mostly larval stages of small fish).
- Acorn worms - "half chordate"; have a reduced tubular dorsal nerve cord and gill slits, but lack a notochord and post anal tail; have vermiform (worm like) bodies; are benthic in shallow waters, usually; some are colonial and live in secreted tubes, similar to those of polychaete worms; most are sedentary or sessile. Their distribution is worldwide, but they are relatively reclusive and difficult to find. Due to presence of small mouth-associated, string-like structure (stomochord), they were originally classified as chordates. This structure was determined to be non-homologous to the notochord of true chordates and resulted in their being reclassified into a distinct phylum of their own.