Marine Phyla Pages -- Coastal Carolina University 

HEMICHORDATA

Hemichordata

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Defining Characteristics

 

Hemichordates are distinguished by a tripartite (threefold) division of the body.  Not familiar creatures to most people, hemichordates form a small phylum (only a few hundred species). Their importance for the study of vertebrate evolution, however, cannot be underestimated. The fossil record of one group of hemichordates, the graptolites, is very well known and is often used to correlate rocks.’

 

Systematics

 

At the forward end of the body is a preoral lobe, behind this is a collar, and last comes a trunk. The name "hemichordate" means "half chordate," and hemichordates share some (but not all) of the typical chordate characteristics. There are branchial openings, or "gill slits," that open into the pharynx; there is a rudimentary structure in the collar region, the stomochord, that is similar to a notochord; and there is a dorsal nerve cord, in addition to a smaller ventral nerve cord. However, hemichordates are not classified as true chordates, although they are quite closely related. Some DNA-based studies of evolution suggest that hemichordates are actually closer to echinoderms than to true chordates. This is supported by the fact that the larvae of at least some hemichordates look very much like those of some echinoderms.

 

Of the three classes of hemichordates, the most familiar living ones are the Enteropneusta, the acorn worms.  Acorn worms also have multiple branchial openings, as many as 200 in some species.

 

Feeding & Locomotion

 

They are slow burrowers, using the proboscis to burrow through sediment, and may either deposit feed (consume sediment and digest the organic matter, rather like earthworms in soil) or suspension feed (collect suspended particles from the water). Some of these worms may be very large; one species may reach a length of 2.5 meters (almost eight feet), although most are much smaller.

 

Hemichordata Links

 

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