This group is sometimes considered the sister group of the echinoderms. These animals are worm-shaped and have a stomochord; which is thought to be a precursor to the notochord (a flexible backbone-like structure). This group includes the extinct graptolites and the modern acorn worm.

Above: Acorn worm

For more information:

http://www.ucmp.berkeley.edu/chordata/hemichordata.html

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/GRAPTO/graptoweb/index_files/index.html