Acorn worm
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name.[1] The closest non-hemichordate relatives of the Enteropneusta are the echinoderms.[2] There are 111 known species of acorn worm in the world,[3] the main species for research being Saccoglossus kowalevskii. Two families—Harrimaniidae and Ptychoderidae—separated at least 370 million years ago.[4]
Until recently, it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the early 21st century 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.[5][6][7][8][9] 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.[10] The body length normally range from 2 centimetres (0.79 in) to 2.5 metres (8 ft 2 in) (Balanoglossus gigas),[11] but one species, Meioglossus psammophilus, only reach 0.6 millimetres (0.024 in).[12] Due to secretions containing elements like iodine, the animals have an iodoform-like smell.[13]
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.[15]
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.[15]
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.[16] Research indicates that the rate of feeding of acorn worms that are detritus feeders is dependent on food availability and flow rate.[17] 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.[15]
The mouth cavity is tubular, with a narrow diverticulum or stomochord extending up into the proboscis. This diverticulum was once thought to be homologous with the notochord of chordates, hence the name "hemichordate" for the phylum. The mouth opens posteriorly into a pharynx with a row of gill slits along either side. The remainder of the digestive system consists of an oesophagus and intestine; there is no stomach.[15]
