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| Hemichordata |
| The Hemichordates (Graptolites) are divided into two classes. They construct burrows, commonly U-shaped, in sandy or muddy seafloors, using an streachable, muscular arm attached to a thick ring that resembles an acorn--hence the name acornworms. They secrete a slime that collects food particles, but some species filter sediments and sand through a complicated pharynx with many gill slits. They are worm-shaped. The body is much simple and gill slits are reduced to one pair or none. |
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