Enteropneusta (order)
| Acorn worms Temporal range: 505–0 Ma |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Hemichordata |
| Class: |
Enteropneusta Gegenbaur, 1870 |
| Order: | Enteropneusta |
| Families | |
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. They are closely related to the echinoderms. There are about 90 species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii.
Until recently, it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the last decade has seen the description of a new family, the Torquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites. It is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia, but that they eventually started to live a safer and more sheltered existence in sediment burrows instead. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8.2 ft), although most acorn worms are much smaller.
Most acorn worms range from 9 to 45 centimetres (3.5 to 17.7 in) in length, with the largest species, Balanoglossus gigas, reaching 1.5 metres (5 ft) or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk. The creature's mouth is located at the collar behind the proboscis.
The skin is covered with cilia as well as glands that secrete mucus. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly, using ciliary action and peristalsis of the proboscis.
Many acorn worms are detritus feeders, eating sand or mud and extracting organic detritus. Others feed on organic material suspended in the water, which they can draw into the mouth using the cilia on the gill bars. A groove lined with cilia lies just in front of the mouth and directs suspended food into the mouth and may allow the animal to taste.
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Wikipedia