Classification of Hemichordata

Phylum Hemichordata is divided into four classes:

Enteropneusta

• This class contains acorn worms.
• They have a vermiform body and are located on sandy beaches near seas in warm climates. 
• This class contains about 70 species. 
• They are known as acorn worms because of their proboscis and collar appearance.

Pterobranchia

• They live in deep sea waters and have tube-dwelling. 
• They attach to other organisms for their survival, as they are bottom dwellers. 
• This class contains about 20 species.

Order 1 Cephalodiscida

• Single paired gill slits.
• Single-paired gonads.
• Several individuals live unconnected in a common jelly-like house.
• Examples: Cephalodiscus and Atubaria.

Order 2 Rhabdopleurida

• Single gonad is present.
• Gill slits are present.
• Colonial, zooids attached by a stolon.
• Example: Rhabdopleura (single genus).

Planctosphaeroidea

• Planctosphaeroidea is known only from a few floating larvae.
• The L-shaped alimentary canal.
• Larval body is covered by extensively branched ciliary bands.

Graptolita

• These are extinct colonial hemichordates, mainly known from the fossil structures of their tubes.
• Each animal is housed in a zooid. These were plentiful in the Ordovician and Silurian periods.
• Example: Dendrograptus.

There are distinct animals with distinct structures and forms. Till now, a million species of animals have been introduced, and the urge for classification becomes all the more essential. The classification also assists in allowing a systematic position for newly introduced species. Animals have definite shapes and sizes. They are unbranched except for the sponges. Animal organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc. 

The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They have sense organs, nervous systems, and excretory organs. There are different Phyla such as Porifera, Cnidaria, Ctenophora, Platyhelminthes, Nematode, Annelida, Arthropoda, Mollusca, Echinodermata, hemichordate, and Chordata. Let us have an overview of Phylum Hemichordata.