Hemichordata

(systematics) A group of marine animals categorized as either a phylum of deuterostomes or a subphylum of chordates; includes the Enteropneusta, Pterobranchia, and Graptolithina.

(Hemichordates)

Phylum: Hemichordata

Number of families: 8

Thumbnail description
Small wormlike marine animals that live individually or in colonies, depending on the species

Evolution and systematics

The extant members of the phylum Hemichordata (formerly called Stomochordata) number about 92 and are typically divided into three classes:

• Enteropneusta or acorn worms: four families;
• Pterobranchia or pterobranchs: three families; and
• Planctosphaeroidea: one family.

Hemichordates are closely related to the chordates, sharing many chordate or chordate-like features. In the enteropneusts, these features include lateral openings, which are pharyngeal gill pores that connect the pharynx or airway to the exterior; and a stomochord that is somewhat analogous to the chordate notochord, although it has been described as "no more than an anterior extension of the buccal cavity." Despite these similarities, genetic studies, morphological comparisons of the larvae and anatomy of the nervous system, and biochemical evidence have recently indicated that hemichordates may be more closely related to echinoderms (starfishes and sea urchins) than to chordates.

In addition, recent genetic analyses of the extant hemichordates suggest that the pterobranchs are not plesiomorphic (similar in form) as previously thought, but have actually evolved from a forerunner to the enteropneusts. Under this arrangement, the lineage of the Ptychoderidae family within the enteropneusts split off first in the course of evolution, followed by a later separation of the Harrimaniidae family of the enteropneusts and the pterobranchs.

In addition to these three extant classes of Hemichordata, there is an extinct class, Graptolithina, known from fossils found in rocks dating from the Ordovician and Silurian periods (505–410 million years ago).

Physical characteristics

Hemichordates have two major body plans. One of the most notable characteristics of the enteropneustan hemichordates is their three-part body plan, which includes a protosome, or anterior proboscis (sometimes called a pre-oral lobe); followed by a mesosome or collar; and finally a meta-some, or trunk. Cilia, which are present over all body areas, play roles in locomotion and in distributing the proteinaceous mucus secreted by the acorn worms. The largest species is the enteropneust Balanoglossus gigas of Brazil, an acorn worm that reaches 4.9 ft (1.5 m) in length and lives in long burrows stretching over more than 9.8 ft (3 m).

Pterobranchs have a three-part body plan like the enteropneusts, but with a shorter, shield-shaped proboscis and a more complex collar. In some species, the collar has tentacled arms. Pterobranchs form colonies, often with individuals attached by so-called stolons or stems. The individual animals are called zooids and are quite small, typically less than 0.04 in (1 mm) long. Groups form and live within a coenecium, which is a network of proteinaceous tubes built with secretions from each animal's proboscis.

The class Planctosphaeroidea has only one species, Planctosphaera pelagica, and it is known only from its larvae. Although several times larger at 0.3–1 in (8–25 mm) long, the almost-spherical, transparent P. pelagica larva is otherwise quite similar to enteropneust tornaria, having a gelatinous body covered with cilia. Unlike tornaria, however, the epidermis of P. pelagica has two deep invaginations (pouchlike formations) as well as numerous glands that secrete mucus.

Distribution

Enteropneusts, pterobranchs, and planctosphaeroids occur in oceans throughout the world. In general, the acorn worms live in shallower areas and the pterobranchs in deeper waters. The single species known from the class Planctosphaeroidea is found in both the Atlantic and Pacific Oceans at depths between 246 ft (75 m) and about 3,280 ft (1,000 m).

Habitat

Habitat varies by class. Adult acorn worms are typically found in either intertidal or shallow marine areas, although they are occasionally found in deeper water. They generally inhabit burrows in the sea bottom but also live sometimes in the sand inside shells, under rocks, in thick seaweed, or between root tangles. Adult pterobranchs are colonial forms that live in secreted tubular coenecia, and the planctosphaerids are planktonic.

Behavior

The acorn worms are solitary animals that are generally found sheltered in burrows, under rocks, or in thick vegetation. The burrowing species, like Balanoglossus clavigerus, use their proboscis primarily to fashion U-shaped burrows. They line the burrow walls with epidermal secretions that provide added strength. Each end of the burrow lies at the surface of the sea bottom and the remainder of the "U" is underground. One end is a cone-shaped depression in the sand bottom, and the other can be identified by a several-inch-tall pile of worm castings a short distance away. Besides this main burrow, Balanoglossus also employs a few side tunnels. Frequently, acorn worms will stretch their proboscis and collar out of the tunnel, but they spend the bulk of their time underground. When threatened, acorn worms respond by expanding the proboscis, effectively anchoring the animal in its burrow or tangle of vegetation while withdrawing the rest of the body. Studies of phototaxis (movement toward or away from a light source) reveal that illumination stimulates some species, like Saccoglossus ruber, to burrow deeper.

Because of the burrowing nature of most hemichordates, little is known about the reproductive and other behaviors of many species.

Feeding ecology and diet

Hemichordates may be either suspension- or sediment-feeders. The latter, like Balanoglossus clavigerus, take in sediment and obtain nutrients from the organic matter contained in it. The suspension-feeding adult hemichordates, as well as the filter-feeding tornaria larvae, gather their meals by generating currents with the cilia located on their bodies and drawing in organic matter. There is some uncertainty about the role their mucus plays in prey capture. Some scientists believe food sticks to the mucus-covered proboscis, and the cilia then beat in a pattern that draws the mucus and the food together to the mouth at the bottom of the proboscis. Researchers studying such species as Rhabdopleura normani, on the other hand, have found that normal feeding does not involve mucus; instead, the organism relies on the cilia to change direction in movements called local reversals and thereby directs food particles to its mouth.

Indirect-developing species have free-swimming tornaria larvae that live on plankton for weeks to months. Some species, like Saccoglossus horsti, have free-swimming larvae that obtain all their nutrition from their yolk, and within a few days take up the sessile (permanently attached) lifestyle. Studies of P. pelagica larvae indicate that their mucus may facilitate feeding, although the details are unclear and several alternative hypotheses have been suggested for the mucus.

Reproductive biology

Enteropneusts normally reproduce sexually via external fertilization, and develop either directly or via tornaria larvae. The indirect developers, including Balanoglossus and Ptychodera species, are in the majority. These species develop from egg to planktonic tornaria larva to adult form. The tornaria larvae eventually become sessile, with the burrow-dwellers developing tails behind the anus that they use to anchor themselves in their mucus-lined tunnels. Direct developers, on the other hand, hatch into adult animals, bypassing the planktonic phase. An example is Saccoglossus kowalevskii. Enteropneusts are also known to reproduce asexually by fragmentation of the adult's body, but this mode of reproduction is uncommon. Typically, the females lay up to 3,000 eggs at a time, and the males release sperm that appear to find the eggs by following chemical cues. Reproduction in many species is cyclical. Saccoglossus horsti, for example, breeds in late spring to midsummer. Water temperature and tides appear to affect reproductive timing in hemichordates.

The pterobranchs reproduce via short-lived larvae in a fashion similar to the enteropneusts, but more often resort to reproduction by asexual budding. Many, perhaps all, of the hemichordates are able to regenerate portions of their trunks.

Conservation status

No hemichordates are listed as threatened by the IUCN.

Significance to humans

The hemichordates are perhaps most important to humans for the information they can provide about the origin of chordates, deuterostomes, and bilateral animals.

Species accounts

Resources

Barrington, E. J. W. The Biology of Hemichordata and Protochordata. San Francisco: W. H. Freeman and Co., 1965.

Cameron, C. B., J. R. Garey, and B. J. Swalla. "Evolution of the Chordate Body Plan: New Insights from Phylogenetic Analyses of Deuterostome Phyla." Proceedings of the National Academy of Sciences 97, no. 9 (April 25, 2000): 4469–4474.

Halanych, K. M. "Suspension Feeding by the Lophophore-Like Apparatus of the Pterobranch Hemichordata Rhabdopleura normani." Biology Bulletin 185 (December 1993): 417–427.

Hart, M. W., R. L. Miller, and L. P. Madin. "Form and Feeding of a Living Planctosphaera pelagica (Phylum Hemichordata)." Marine Biology 120 (1994): 521–533.

Lester, S. M. "Ultrastructure of Adult Gonads and Development and Structure of the Larva of Rhabdopleura normani (Hemichordata: Pterobranchia)." Acta Zoologica (Stockholm) 69, no. 2 (1988): 95–109.

British & Irish Graptolite Group. c/o Dr. A. W. A. Rushton, The Natural History Museum, Cromwell Rd., South Kensington, London, SW7 5BD United Kingdom. Web site:

The Graptolite Working Group of the International Palaeontological Association. c/o Dr. Charles E. Mitchell, Department of Geology, State University of New York at Buffalo, Buffalo, NY 14260-3050 United States. E-mail: cem@acsu.buffalo.edu Web site:

"Chris Cameron's Homepage." (15 July 2003). . "Hemichordate Phylogeny." University of Washington Faculty Web Server. (15 July 2003). .

[Article by: Leslie Ann Mertz, PhD]

A group of deuterostome animals that includes the classes Enteropneusta, Pterobranchia, and Planc-tosphaeroidea. The last, a monospecific class, is represented by Planctosphaera pelagica, a planktonic larva that occupies low depths and resembles the larval tornaria of Enteropneusta. See also Enteropneusta; Pterobranchia.

The hemichordates have a slender tubular diverticulum that projects forward into the protosome from the roof of the buccal cavity. That organ, also called a stomochord or buccal diverticulum, is a supporting axial rod of the protosome and resembles the notochord of Chordata, but it does not have the same position, structure, origin, or function, and so they are not homologous. See also Chordata.

The hemichordates are not plentiful animals. All are marine species that live in a wide range of habitats and depths, from intertidal to abyssal, and show a worldwide distribution. They vary in size from a fraction of an inch to 7 ft or more. The members of the phylum differ widely. The enteropneusts are vermiform and solitary, whereas the pterobranchs are sacciform and colonial.

The hemichordates are a primitive group, having a tripartite body and coelom; their embryonic development resembles the echinoderms, with which they also share a primitive nervous system. The Hemichordata, therefore, may be a group at a low level of evolution, between echinoderms and chordates.

Hemichordata Fossil range: Cambrian - Recent
Acorn worm, a hemichordate.
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Deuterostomia
Phylum:	Hemichordata Bateson, 1885
Classes
Enteropneusta Graptolithina (extinct) Pterobranchia Planctosphaeroidea

Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous.

Contents 1 Structure 2 Classification 3 References 4 External links

Structure

They seem to have a primitive form of notochord, formed from a diverticulum of the foregut called a stomochord, but this is most likely the result of convergent evolution rather than homology with the vertebrate notochord. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes.

The bodies of Hemichordates are divided into three parts, proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that cover its inside surface.

Classification

Hemichordata are divided into two classes: the Enteropneusta,^[1] commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic.

References

1. ^ Cannon (2009). "Molecular phylogeny of hemichordata, with updated status of deep-sea enteropneusts". Molecular phylogenetics and evolution 52 (1): 17–24. doi:10.1016/j.ympev.2009.03.027. PMID 19348951.  edit

Cameron, C.B. 2005. A phylogeny of the hemichordates based on morphological characters. Canadian Journal of Zoology. 83(1): 196-2

Winchell, C.J., J. Sullivan, C.B. Cameron, B.J. Swalla, and J. Mallatt. 2002. Evaluating hypotheses of deuterostome phylogeny and chordate evolution with new LSU and SSU ribosomal DNA data. Molecular Biology and Evolution. 19(5): 762-776.

Cameron, C.B., B.J. Swalla and J.R. Garey. 2000. Evolution of the chordate body plan: New insights from phylogenetic analysis of deuterostome phyla. Proceedings of the National Academy of Sciences (USA) 97(9): 4469-4474.

External links

• Cameron, C. B. Evolution and classification of Hemichordata
• At the Tree of Life

Amplexograptus, a graptolite hemichordate, from the Ordovician near Caney Springs, Tennessee.

Wikispecies has information related to: Hemichordata

Wikimedia Commons has media related to: Hemichordata

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