Phylum Hemichordata

Phylum Hemichordata (Greek hemi = half, referring to a once-suspected link to Chordata) includes animals such as Balanoglossus and Saccoglossus. It was once considered a sub-phylum of Chordata, because a structure in the buccal region, the buccal diverticulum, was thought to be a notochord; today it is recognised as a separate phylum of non-chordates.

These animals are exclusively marine, typically living at the bottom of the sea in burrows. Most are free-living, though some, like Rhabdopleura, are sedentary. The body is soft, worm-like (vermiform) and unsegmented, and is divided into three distinct regions: an anterior proboscis, a short collar, and a long trunk. …