Hemichordates
Hemichordates are distinguished by a tripartite (threefold) division of the body. At the forward end of the body is a preoral lobe, behind this is a collar, and last comes a trunk (so 3 segments). Each contains a division of the coelom. The name "hemichordate" means "half chordate," and hemichordates share some (but not all) of the typical chordate characteristics. There are branchial openings, or "gill slits," that open into the pharynx; there is a rudimentary structure in the collar region, the stomochord, that is considered similar to a notochord by some authors (although develops differently from a notochord in the embryo and involves different genes); and there is a dorsal nerve cord, in addition to a smaller ventral nerve cord. However, hemichordates are not classified as true chordates, although they are quite closely related. Some DNA-based studies of evolution suggest that hemichordates are actually closer to echinoderms than to true chordates. This is supported by the fact that the larvae of at least some hemichordates look very much like those of some echinoderms.
Acorn worms
Acorn worms live on the sea-bed, from the shoreline down to the depths of 10,000 ft.. (3,050 m)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 by cilia movements and body contractions. The stomochord provides support for the front of the animal and the coelom acts as a hydrostatic skeleton for the back. They are filter feeders.
The sexes are separate. Eggs and sperm are merely shed into the water and fertilization occurs by chance. Most eggs are very small and they produce small Larvae, which swim freely in the plankton for a short while before settling on the sea-bed.
In Hawaii, large acorn worms grow to 1 inch in diameter and are 18 inches long. They live in U-shaped burrows 30 feet or deeper. There, the big worms swallow large quantities of sand, digest any plant or animal material in it and then pass the sifted sand out behind. These fecal mounds look like coiled ropes of extremely fine sand. At a touch, the piles disperse like ash.
They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish.
Pterobranchs are also considered hemichordates
Pterobranchs are colonial hemichordates living in secreted tubular tubes. They are typically found in deep waters around Antarctica. They reproduce by short-lived planula shaped larvae or asexual budding. Pterobranchs were originally classified similarly to bryozoans and phoronids based on lophophore feeding structures. Molecular data on relationships (DNA sequences) suggest that the similarities are due to convergence.
Are the lophophores of pterobranchs genetically homologous to the lophophores of other clades such as Bryozoans? These structures were re-studied after their affinities to other Hemichordates were recognized. They probably arise from the same basic patterning "genes". However, pterobranchs share stomochords, gill slits, a dorsal and ventral nerve chord and other structures with acorn worms. Hemichordates clearly are related to other deuterostomes and so pterobranchs are now not considered part of the Lophophora.
They may be descendents of graptolites. These are fossils whose affinities are unknown but date from the mid-Cambrian.
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Xenoturbellids are small ciliated worms. They were at first thought to be related to Mollusks. Now know they eat mollusks. New gene sequence analysis places them closer to echinoderms. They have a micro RNA that was previously only known to exist in echinoderms and acorn worms. Moreover, all of the animals analyzed up to now from the new Xenacoelomorpha phylum have the gene RSB66, which could previously only be demonstrated in deuterostomes. Body structure resembles that of a acoel flatworm. No organs except a statocyst. Its position as a stem or "ancestral form" to the deuterostomes is still debated; it may simply be an advanced form of some unknown clade.
