The following points highlight the two main classes of phylum hemichordata. The classes are: 1. Enteropneusta 2. Pterobranchia.
Class # 1. Enteropneusta:
1. Worm-like, sand-burrowing forms, range in length from 25 to 2,500 mm.
2. They are found in shallow waters, although some live at a depth of 4,500 metre.
3. Body divided into proboscis, collar and trunk, the latter being further divided into an anterior branchial region, a middle genital region and a posterior abdominal region terminating into anus.
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4. The mouth is always wide open and capable of closing completely, lies on the ventral surface and its lips are the ventral edges of the collar region.
5. Both proboscis and collar are hollow and their cavities communicate with the exterior by pores that permit water to be taken in or expelled out. The coelom in the trunk region is divided into two halves and without any pore.
6. Series of gill slits, even up to 700 pairs in some species, present on both lateral sides of pharynx extending far backwards in the trunk.
7. A thick-walled, hollow diverticulum of the dorsal wall of the mouth cavity extends forwards into the hollow proboscis.
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8. The nervous system is constructed on the nerve-net principle and consists of a plexus of cells and fibres from the so called dorsal tubular nerve cord in the collar region.
9. Larva Tornaria, similar to the Bipinnaria larva of starfish.
Examples: Balanoglossus, Dolichoglossus, Glossobalanus, Ptychodera, Saccoglossus, etc.
Class # 2. Pterobranchia:
1. Nearly all members are marine, sedentary, colonial, forms of minute size.
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2 – Body divided into three regions—a proboscis with proboscis cavity, a collar with two collar cavities and a trunk with two distinct lateral cavities.
3. Buccal diverticulum and so-called nerve cord similar to those of Enteropneusta.
4. The alimentary canal ‘U’ shaped, anal opening lies near the mouth.
5. Presence of arms bearing tentacles arising from collar.
6. Gill pores single pair in Cephalodiscus, absent in Rhabdopleura (except a pair of ciliated grooves).
7. In most, sexes are separate, but Cephalodiscus is hermaphrodite.
8. Development is direct.
Examples: Cephalodiscus, Rhabdopleura, Atubaria, etc..