- Name Meaning: Half cord
- English Common Name: Acorn worms, hemichordates
- Major distinguishing characteristics: Stomochord in collar
- Approximate number of species described: about 100 living species
The Hemichordata are mostly solitary, though a few are colonial.
They are generally worm-shaped, and live in burrows. They are deposit feeders, filter feeders, or free living detritivores.
There are 2 recognized classes within the Hemichordata.
- Class Pterobranchia
- Class Enteropneusta
Additionally the species Planctosphaera pelagica is known from juveniles. It is significantly different from others, but has not be assigned to a class.
Worm-shaped, the Hemichordata have three main body parts: an anterior proboscis, an intermediate collar, and a posterior trunk.
The proboscis is muscular and ciliated and used in locomotion. It also serves in the collection and transport of food particles to the mouth. The mouth is between the proboscis and the collar.
The trunk contains the pharynx, which has gill slits (or pharyngeal slits), the esophagus, an intestine, the gonads, and the anus.
The Fossil Record
The earliest fossils of hemichordates have been found in the Lower or Middle Cambrian.