Phylum Hemichordata has been divided into following four classes:
Class 1. Enteropneusta:
1. Commonly known as “acorn” or “tongue worms”.
2. Solitary and burrowing worm-like marine animals.
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3. Body consists of proboscis, collar and trunk; collar without tentaculated arms.
4. Alimentary canal straight; mouth and anus at opposite ends.
5. Numerous pairs of U-shaped gill-slits.
6. Two pairs of hepatic caeca present in the middle of the trunk.
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7. Sexes separate; gonads numerous, sac-like.
8. Development with or without tornaria larva.
Examples- Balanoglossus, Saccoglossus (= Dolichoglossus), Ptychodera.
Class 2. Pterobranchia:
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1. Sedentary, solitary or colonial, tubicolous marine animals.
2. Proboscis with ciliated tentacles to produce ciliary feeding currents of water.
3. Collar with two or more tentaculated arms bearing tentacles.
4. One pair of gill-slits or none, never U-shaped.
5. Alimentary canal U-shaped with dorsal anus situated near the mouth at the same end.
6. Sexes separate or united; single or one pair of gonads.
7. Development direct, may or may not include a free swimming larval stage.
8. Asexual reproduction by budding in some.
Order 1. Cephalodiscida:
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1. Solitary or several individuals living unconnected in a common gelatinous house.
2. Collar with several tentaculated arms.
3. Gill-slits single pair.
4. Gonads single pair.
Examples- Cephalodiscus, Atubaria.
Order 2. Rhabdopleurida:
1. Colonial, zooids connected by a stolon.
2. Collar with two tentaculated arms.
3. Gill-slits absent.
4. Gonad single.
Examples- Rhabdopleura (Single genus).
Class 3. Planctosphaeroidea:
This class is represented by a few small rounded, transparent and pelagic larvae, supposed to be specialised tornaria of some unknown hemichordate Planctosphaera pelagica. The larva has branching arborescent ciliated bands on the surface. The alimentary canal of larva is U-shaped. The adult form is yet unknown.
Class 4. Graptolita:
These are extinct colonial hemichordates, mainly known from the fossil structures of their tubes. Each animal is housed in a zooid. These were abundant in the Ordovician and Silurian periods.
Example- Dendrograptus.