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Hermichordates

Hemichordata

Brief Summary

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The phylum Hemichordata includes around 90 described species of enteropneusts (acorn worms) and around 30 described pterobranchs. Like chordates, hemichordates are deuterostomes with pharyngeal gill slits and most have a dorsal (and sometimes hollow) nerve cord. However, they lack a notochord. As adults, all or nearly all hemichordates are benthic (bottom-dwelling) marine animals.

Acorn worms range in size from a few centimeters to over 2 meters. They typically live buried in soft sand or mud, among algal holdfasts, or under rocks. Although they are largely intertidal, a few deep sea species are known. Most pterobranchs live in small colonies of individual zooids within a secreted tube on the ocean floor at depths of 5 to 5000 m. In some species the zooids are connected to each other by tissue extensions called stolons. A long contractile stalk allows the animal to retreat into its tube. The individual zooids are small, rarely exceeding 1 cm in length, but colonies may measure 10 cm or more across. (Brusca and Brusca 2003; Cannon et al. 2009; Margulis and Chapman 2010)

Hemichordates are dioecious (i.e., there are separate males and females, although they cannot be distinguished externally) with external fertilization and, in general, indirect development (i.e., there is a distinct larval form). Asexual reproduction occurs in at least some acorn worms and in most pterobranchs. Acorn worms fragment small pieces from the trunk, each of which can then grow into a new individual. Pterobranch colonies are derived by budding of a single sexually produced individual. Sexual reproduction in pterobranchs produces non-feeding larvae that are brooded in the colony. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein)

Hemichordates are sessile (attached to the substrate) or capable of only limited movement. Feeding habits of hemichordates vary. Acorn worms that burrow in soft sediments are largely direct deposit feeders, digesting organic material ingested with the substrate as they burrow. Other acorn worms are filter feeders, selectively trapping suspended organic particles from the water with their mucus-covered proboscis. Pterobranchs use small dorsally located tentacle-bearing "arms" to capture suspended organic particles. The tentacles on adjacent arms cross to form a lattice across which a mucus net is secreted. Food is trapped in the mucus and moved to the mouth by cilia on the tentacles and arms (and, in at least some species, on the entire body). (Brusca and Brusca 2003)

The evolutionary relationships among the deuterostome phyla (Echinodermata, Hemichordata, Chordata, and possibly Xenoturbellida) remain somewhat controversial (see General Description).

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Comprehensive Description

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The phylum Hemichordata includes around 90 described species of enteropneusts (acorn worms) and around 30 described pterobranchs (see the online Hemichordata World Database). Like chordates, hemichordates are deuterostomes with pharyngeal gill slits and most have a dorsal (and sometimes hollow) nerve cord. However, they lack a notochord. As adults, all or nearly all hemichordates are benthic (bottom-dwelling) marine animals.

Acorn worms range in size from a few centimeters to over 2 meters. They typically live buried in soft sand or mud, among algal holdfasts, or under rocks. Although they are largely intertidal, a few deep sea species are known. Most pterobranchs live in small colonies of individual zooids within a secreted tube on the ocean floor at depths of 5 to 5000 m. In some species the zooids are connected to each other by tissue extensions called stolons. A long contractile stalk allows the animal to retreat into its tube. The individual zooids are small, rarely exceeding 1 cm in length, but colonies may measure 10 cm or more across. (Brusca and Brusca 2003; Cannon et al. 2009; Margulis and Chapman 2010) Although the Hemichordata are traditionally divided into two classes--the solitary, free-living Enteropneusta (acorn worms) and the colonial, tube-dwelling Pterobranchia--recent phylogenetic studies suggest that the pterobranch clade may actually be nested within the acorn worms (Cannon et al. 2009 and references therein).

The basic body plan of an acorn worm includes a short, often conical, proboscis; a collar, which bears the ventral mouth at its anterior end; and a long trunk with the anus situated at its posterior end (although harrimaniid enteropneusts have a post-anal tail as juveniles). In most species, there is a clear region of the trunk that is chacterized by the presence of numerous gill pores. In contrast, pterobranchs are colonial, usually pear-shaped or globular tiny animals living in secreted tubes. The pterobranch gut is U-shaped, and individual zooids suspension-feed with ciliated tentacles. Members of the genus Cephalodiscus possess a single pair of gill slits, whereas Rhabdopleura have no gill slits. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein; Margulis and Chapman 2010)

Hemichordates are dioecious (i.e., there are separate males and females, although they cannot be distinguished externally) with external fertilization and, in general, indirect development (i.e., there is a distinct larval form). Asexual reproduction occurs in at least some acorn worms and in most pterobranchs. Acorn worms fragment small pieces from the trunk, each of which can then grow into a new individual. Pterobranch colonies are derived by budding of a single sexually produced individual. Sexual reproduction in pterobranchs produces non-feeding larvae that are brooded in the colony. (Brusca and Brusca 2003; Cannon et al. 2009 and references therein)

Hemichordates are sessile (attached to the substrate) or capable of only limited movement. Feeding habits of hemichordates vary. Acorn worms that burrow in soft sediments are largely direct deposit feeders, digesting organic material ingested with the substrate as they burrow. Other acorn worms are filter feeders, selectively trapping suspended organic particles from the water with the proboscis. These particles may then be trapped in mucus secreted over the surface of the proboscis and moved posteriorly by ciliary currents. Pterobranchs use small dorsally located tentacle-bearing "arms" to capture suspended organic particles. The tentacles on adjacent arms cross to form a lattice across which a mucus net is secreted. Food is trapped in the mucus and moved to the mouth by cilia on the tentacles and arms (and, in at least some species, on the entire body). (Brusca and Brusca 2003)

The Hemichordata share some characters, such as pharyngeal gill slits or pores, that have been shown to be homologous with those of chordates. However, a range of molecular, developmental, and morphological data indicate that hemichordates and echinoderms, not chordates, are sister groups, together comprising the Ambulacraria. Given that hemichordates are more closely related to echinoderms than to chordates, any truly homologous features shared between hemichordates and chordates must have been present in the most recent common ancestor of deuterostomes. (Swalla and Smith 2008 and references therein; Cannon et al. 2009 and references therein)

The deuterostome phyla include Echinodermata, Hemichordata, and Chordata (and possibly Xenoturbellida, a clade that is apparently sister to the Ambulacraria, but alternatively may be sister to all other deuterostomes [Swalla and Smith 2008] or even all other Bilateria [Hejnol et al., 2009; Nielsen 2010]). Chordata has been considered to be composed of three subphyla: Vertebrata, Cephalochordata (Branchiostoma and Epigonichthyes), and Urochordata (Tunicata). Cameron et al. (2000) investigated phylogenetic relationships within the deuterostomes using an 18S ribosomal DNA data set. They concluded that the deuterostomes are composed of two major clades: chordates and (echinoderms + hemichordates). Their analysis strongly supported the monophyly of each of four major deuterostome taxa: (Vertebrata + Cephalochordata), Urochordata, Hemichordata, and Echinodermata. Based on their analysis, in combination with morphological and life history data, the authors concluded that Urochordata should be treated as a phylum, rather than a subphylum within Chordata, that is sister to the Chordata (Vertebrata + Cephalochordata). This grouping of vertebrates with cephalochordates is consistent with the traditional view of deuterostome relationships, a view that has been challenged by a number of more recent studies, which recover vertebrates and tunicates as sister taxa (e.g., see review by Stach 2008 and references therein; Swalla and Smith (2008) note that mitochondrial and ribosomal evidence place cephalochordates as sister group to the vertebrates, whereas genomic evidence places tunicates as the sister group to the vertebrates). Their analysis also suggested that the pterobranchs are neither sister to the enteropneusts nor are they basal deuterostomes, as had been suggested in the past, but rather that the pterobranchs are nested within the enteropneusts. (Cameron et al. 2000)

Lambert (2005a,b) reviewed the history of research on hemichordates, cephalochordates, and tunicates as well as diverse aspects of their biology.

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Hemichordate

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Hemichordate is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The extinct class Graptolithina is closely related to the pterobranchs.[1]

Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest secreted tubes)[2] and are deposit feeders, but some species are pharyngeal filter feeders, while the family Torquaratoridae are free living detritivores. Many are well known for their production and accumulation of various halogenated phenols and pyrroles.[3] Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium.[4]

Anatomy

The body plan of hemichordates is characterized by a muscular organization. The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome.

The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles. The mouth is located between the proboscis and the collar. The trunk is the longest part of the animal. It contains the pharynx, which is perforated with gill slits (or pharyngeal slits), the esophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile member of the acorn worm family Harrimaniidae.[5]

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Anatomy of Saccoglossus kowalevskii[6]

The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium. The mesosome extends into one pair (in the genus Rhabdopleura) or several pairs (in the genus Cephalodiscus) of tentaculated arms used in filter feeding. The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of the colony, produced by asexual budding. In the genus Cephalodiscus, asexually produced individuals stay attached to the contractile stalk of the parent individual until completing their development. In the genus Rhabdopleura, zooids are permanently connected to the rest of the colony via a common stolon system.

They have a diverticulum of the foregut called a stomochord, previously thought to be related to the chordate notochord, but this is most likely the result of convergent evolution rather than a homology. A hollow neural tube exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes[citation needed].[7]

Some species biomineralize in calcium carbonate.[8]

Development

Together with the Echinoderms, the hemichordates form the Ambulacraria, which are the closest extant phylogenetic relatives of chordates among the invertebrates. Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly.[9] Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm.

The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm.[10] The Pterobranch genus most extensively studied is Rhabdopleura from Plymouth, England and from Bermuda.[11][12][13][14]

The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.

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Schematic of embryonic cleavage and development in P.flava and S.kowalevskii

Ptychodera flava

P. flava’s early cleavage pattern is similar to that of S. kowalevskii. The first and second cleavages from the single cell zygote of P. flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. P. flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and 4 larger macromeres. Further divisions occur until P. flava finishes the blastula stage and goes on to gastrulation. The animal mesomeres of P. flava go on to give rise to the larva’s ectoderm, animal blastomeres also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues.[15] Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development P. flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage.[16]

Saccoglossus kowalevskii

Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go thru gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm[17][18]

Genetic control of dorsal-ventral hemichordate patterning

Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii, and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4, which is homologous to Drosophila’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on the ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii, such as a netrin that groups with netrin gene class 1 and 2.[6] Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.

Classification

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

Hemichordata are divided into two classes: the Enteropneusta,[19] 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 120 living species.[20] 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. It is possible that the extinct organism Etacystis is a member of the Hemichordata, either within or with close affinity to the Pterobranchia.[21]

There are 130 described species of Hemichordata and many new species are being discovered, especially in the deep sea.[22]

Phylogeny

A phylogenetic tree showing the position of the hemichordates is:

.mw-parser-output table.clade{border-spacing:0;margin:0;font-size:100%;line-height:100%;border-collapse:separate;width:auto}.mw-parser-output table.clade table.clade{width:100%}.mw-parser-output table.clade td{border:0;padding:0;vertical-align:middle;text-align:center}.mw-parser-output table.clade td.clade-label{width:0.8em;border:0;padding:0 0.2em;vertical-align:bottom;text-align:center}.mw-parser-output table.clade td.clade-slabel{border:0;padding:0 0.2em;vertical-align:top;text-align:center}.mw-parser-output table.clade td.clade-bar{vertical-align:middle;text-align:left;padding:0 0.5em}.mw-parser-output table.clade td.clade-leaf{border:0;padding:0;text-align:left;vertical-align:middle}.mw-parser-output table.clade td.clade-leafR{border:0;padding:0;text-align:right} Deuterostomia Chordata

Cephalochordata Branchiostoma lanceolatum (Pallas, 1774).jpg

Olfactores

Urochordata Tunicate komodo.jpg

   

Vertebrata/Craniata Cyprinus carpio3.jpg

      Ambulacraria

Echinodermata Portugal 20140812-DSC01434 (21371237591).jpg

   

Hemichordata Balanoglossus by Spengel 1893.png

     

The internal relationships within the hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[23]

Hemichordata Pterobranchia

Cephalodiscidae Cephalodiscus dodecalophus McIntosh.png

   

Rhabdopleuridae Rhabdopleura normani Sedgwick.png

    Enteropneusta

Harrimaniidae

     

Spengelidae

Ptychoderidae

Balanoglossus by Spengel 1893.png

   

Torquaratoridae

         

References

  1. ^ Sato, Atsuko; Rickards RB; Holland PWH (December 2008). "The origins of graptolites and other pterobranchs: a journey from 'Polyzoa'". Lethaia. 41 (4): 303–316. doi:10.1111/j.1502-3931.2008.00123.x..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""'"'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
  2. ^ Caron, J. B.; Morris, S. C.; Cameron, C. B. (2013). "Tubicolous enteropneusts from the Cambrian period". Nature. 495 (7442): 503–506. Bibcode:2013Natur.495..503C. doi:10.1038/nature12017. PMID 23485974.
  3. ^ Giray, Cem; G.M. King (1997). "Predator deterrence and 2,4-dibromophenol conservation by the enteropneusts, Saccoglossus bromophenolosus and Protoglossus graveolens". Marine Ecology Progress Series. 159: 229–238. Bibcode:1997MEPS..159..229G. doi:10.3354/meps159229.
  4. ^ Sato, Atsuko; Bishop JDD; Holland PWH (2008). "Developmental biology of pterobranch hemichordates: history and perspectives". Genesis. 46 (11): 587–91. doi:10.1002/dvg.20395. PMID 18798243.
  5. ^ Tassia, MG; Cannon, JT; Konikoff, CE; Shenkar, N; Halanych, KM; Swalla, BJ. "The Global Diversity of Hemichordata". PLoS One. 11: e0162564. Bibcode:2016PLoSO..1162564T. doi:10.1371/journal.pone.0162564. PMC 5049775. PMID 27701429.
  6. ^ a b Lowe, C J; Terasaki, M; Wu, M; Freeman Jr, R M; Runft, L; Kwan, K; Gerhart, J (22 August 2006). "Dorsoventral patterning in hemichordates: insights into early chordate evolution". PLoS Biology. 4 (9): e291. doi:10.1371/journal.pbio.0040291.
  7. ^ Nomakstainsky, M; et al. (11 August 2009). "Centralization of the deuterostome nervous system predates chordates". Current Biology. 19 (15): 1264–9. doi:10.1016/j.cub.2009.05.063. PMID 19559615.
  8. ^ Cameron, C. B.; Bishop, C. D. (2012). "Biomineral ultrastructure, elemental constitution and genomic analysis of biomineralization-related proteins in hemichordates". Proceedings of the Royal Society B: Biological Sciences. 279 (1740): 3041–3048. doi:10.1098/rspb.2012.0335. PMC 3385480. PMID 22496191.
  9. ^ Lowe, CJ; Tagawa, K; Humphreys, T; Kirschner, M; Gerhart, J (2004). "Hemichordate embryos: procurement, culture, and basic methods". Methods Cell Biol. 74: 171–94. PMID 15575607.
  10. ^ Tagawa, K.; Nishino, A; Humphreys, T; Satoh, N. (1 January 1998). "The Spawning and Early Development of the Hawaiian Acorn worm (Hemichordate), Ptycodhera flava". Zoological Science. 15: 85–91. doi:10.2108/zsj.15.85. PMID 18429670.
  11. ^ Stebbing, ARD (1970). "Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)". Marine Biology. 5 (3): 205–212. doi:10.1007/BF00346908.
  12. ^ Dilly, PN (January 1973). "The larva of Rhabdopleura compacta (Hemichordata)". Marine Biology. 18: 69–86. doi:10.1007/BF00347923.
  13. ^ Lester, SM (June 1988). "Settlement and metamorphosis of Rhabdopleura normani (Hemichordata: Pterobranchia)". Acta Zoologica (Stockholm). 69 (2): 111–120. doi:10.1111/j.1463-6395.1988.tb00907.x.
  14. ^ Lester, SM (1986). "Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleura normani". Acta Zoologica (Stockholm). 69 (2): 95–109. doi:10.1111/j.1463-6395.1988.tb00906.x.
  15. ^ Henry, JQ; Tagawa, K; Martindale, MQ (November–December 2001). "Deuterostome evolution: early development in the enteropneust hemichordate, Ptychodera flava". Evolution & Development. 3 (6): 375–90. doi:10.1046/j.1525-142x.2001.01051.x. PMID 11806633.
  16. ^ Colwin, A; Colwin, L (1950). "The developmental capacities of separated early blastomeres of an enteropneust, Saccoglossus kowalevskii". Journal of Experimental Zoology. 155: 263–296. doi:10.1002/jez.1401150204.
  17. ^ Colwin, A; Colwin, L (1951). "Relationships between the egg and larva of Saccoglossus kowalevskii (Enteropneusta): axes and planes; general prospective significance of the early blastomeres". Journal of Experimental Zoology. 117: 111–138. doi:10.1002/jez.1401170107.
  18. ^ Colwin, Arthur L.; Colwin, Laura Hunter (May 1953). "The normal embryology of saccoglossus kowalevskii (enteropneusta)". Journal of Morphology. 92 (3): 401–453. doi:10.1002/jmor.1050920302.
  19. ^ Cameron, CB; Garey, JR; Swalla, BJ (25 April 2000). "Evolution of the chordate body plan: new insights from phylogenetic analyses of deuterostome phyla". Proceedings of the National Academy of Sciences of the United States of America. 97 (9): 4469–74. Bibcode:2000PNAS...97.4469C. doi:10.1073/pnas.97.9.4469. PMC 18258. PMID 10781046.
  20. ^ Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
  21. ^ "Etacystis communis, a Fossil of Uncertain Affinities from the Mazon Creek Fauna (Pennsylvanian of Illinois)". Journal of Paleontology. 50: 1157–1161. November 1976.
  22. ^ Tassia, MG; Cannon, JT; Konikoff, CE; Shenkar, N; Halanych, KM; Swalla, BJ (2016). "The Global Diversity of Hemichordata". PLoS ONE. 11 (10): e0162564. Bibcode:2016PLoSO..1162564T. doi:10.1371/journal.pone.0162564. PMC 5049775. PMID 27701429.
  23. ^ Tassia, Michael G.; Cannon, Johanna T.; Konikoff, Charlotte E.; Shenkar, Noa; Halanych, Kenneth M.; Swalla, Billie J. (2016-10-04). "The Global Diversity of Hemichordata". PLoS ONE. 11 (10): e0162564. Bibcode:2016PLoSO..1162564T. doi:10.1371/journal.pone.0162564. PMC 5049775. PMID 27701429.

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Hemichordate: Brief Summary

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Hemichordate is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The extinct class Graptolithina is closely related to the pterobranchs.

Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest secreted tubes) and are deposit feeders, but some species are pharyngeal filter feeders, while the family Torquaratoridae are free living detritivores. Many are well known for their production and accumulation of various halogenated phenols and pyrroles. Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium.

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Habitat

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Hemichordates have been found living in a wide variety of depths and habitats.
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WoRMS Editorial Board
bibliographic citation
Bateson W (1885) The later stages in the development of Balanoglossus kowalevskii, with a suggestion as to the affinities of the Enteropneusta. Quarterly Journal of Microscopical Science 25: 81-122. Hou XG, Aldridge RJ, Siveter DJ, Williams M, Zalasiewicz J, et al. (2011). An early cambrian hemichordate zooid. <em>Curr Biol.</em> 21, 612-616.
i18n: Contributor
Charlotte Konikoff [email]

IUCN Red List Category

provided by World Register of Marine Species
Not Evaluated
license
cc-by-4.0
copyright
WoRMS Editorial Board
bibliographic citation
Bateson W (1885) The later stages in the development of Balanoglossus kowalevskii, with a suggestion as to the affinities of the Enteropneusta. Quarterly Journal of Microscopical Science 25: 81-122. Hou XG, Aldridge RJ, Siveter DJ, Williams M, Zalasiewicz J, et al. (2011). An early cambrian hemichordate zooid. <em>Curr Biol.</em> 21, 612-616.
i18n: Contributor
Charlotte Konikoff [email]