The Hemichordates are separated into three Classes which differ considerably in shape.
They are worm-like bottom dwellers. Some species are colonial, and some live in tubes they have secreted. Most are either sessile or sedentary. They range in length from 0.5 - 250 cm.
These are commonly known as
the acorn worms. There are about 70 species, and are usually less than
2.5 cm in length, though some are larger, see Saccoglossus kowalevskii right.
They are worm-like and mucous covered with a long ciliated proboscis
which is used in food gathering and burrowing. Two rows of pharyngeal slits run
down the trunk, but have no gills attached. Some respiration and gas exchange does occur through these slits, but it also occurs over the entire body surface. They have no lophophore.
They are sluggish movers and live under stones or in burrows in mud or sand, usually in the intertidal zone. Reproduction is both sexual and
asexual (fragmentation), and fertilisation is external.
Saccoglossus kowalevskii, on the right, is common on the Atlantic coasts of Europe and North America.