The Hemichordates, or Acorn Worms and Pterobranchs are a small phylum of worm-shaped marine animals that were once considered to be a part of the Chordates but are now considered to be related to the Echinoderms. The larvae of some hemichordates look very much like those of some echinoderms. There are up to 300 species of Hemichordates.

The bodies of Hemichordates are divided into three parts - proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract but the muscles in their digestive tract are very poorly developed, and food moves through it by using the cilia that cover its insides. They have "gill slits" that open into the pharynx.

They vary widely in size, from up to 2.5 m for the around 90 species of acorn worms to up tp 1 mm for the pterobranchs. These two groups also vary in appearance and behaviour.

Acorn worm
Image © Ria Tan Flickr

Acorn worms (also called the Enteropneusta) are large individuals that build U-shaped burrows in sediments, digesting any organic material in the soil, others feed on suspended particles in the water.

Pterobranchs (only about 20 living species) are quite different from acorn worms. They form large colonies in which each individual is connected to another by stems. These creatures create their own homes, a series of tubes composed of the protein collagen released by glands in their bodies. The proboscis is shield-shaped, not elongated as it is in acorn worm. The second section of the body contains a pair of branched tentacles that are used to collect small food particles from the water.

Pterobranch (Photo by Susie Balser)
Image from www.rvgould.uconn.edu/week4.htm

References

http://en.wikipedia.org/wiki/Hemichordata
www.angelfire.com/mo2/animals1/phylum/hemichord.html
www.ucmp.berkeley.edu/chordata/hemichordata.html
/www.earthlife.net/inverts/hemichordata.html