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Bilateria

(Subkingdom)

Overview

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The Bilateria are all animals having a , i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralians are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges and cnidarians. For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few highly reduced forms, the Bilateria have complete digestive tracts with separate mouth and anus. Most Bilateria also have a type of internal body cavity, called a coelom. It was previously thought that acoelomates gave rise to the other group, but there is some evidence now that in the main acoelomate phyla (flatworms and gastrotrichs) the absence could be secondary.

Evolution

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The hypothetical last common ancestor of all bilaterians is termed the "Urbilaterian".[1][2] There is some debate about its appearance. The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[3] Earlier fossils are controversial; The fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[4] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[5]

Phylogeny

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There are two or more superphyla (main lineages) of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and possibly a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa (molting animals) and Lophotrochozoa. Some taxonomists also recognize another protostome superphylum, Platyzoa, while others would include the Platyzoans in Lophotrochozoa[6] or not include them in any superphylum. The arrow worms (Chaetognatha) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins support protostome affinities.[7][8]

A proposed phylogeny of the Bilateria after Nielsen is as follows.[9] An alternate phylogeny suggests a basal group called the ecdysozoa.

Photos

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Taxonomy

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The Subkingdom Bilateria is a member of the Kingdom Animalia. Here is the complete "parentage" of Bilateria:

The Subkingdom Bilateria is further organized into finer groupings including:

Phyla

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Acanthocephala

The Acanthocephala ( a?a????, akanthos, thorn + ?efa??, kephale, head) is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterised by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans typically have complex life cycles, involving a number of hosts, including invertebrates, fishes, amphibians, birds, and mammals. About 1150 species have been described. [more]

Acoelomorpha

[more]

Annelida

The annelids, collectively called Annelida (from annelés "ringed ones", ultimately from Latin anellus "little ring"), are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments. Although most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans. [more]

Arthropoda

An arthropod is an that has an exoskeleton (external skeleton), a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda (from Greek ?????? arthron, "joint", and p?d?? podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of a-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living species, and are one of only two animal groups that are really successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long. [more]

Brachiopoda

Brachiopods (from brachium, arm + New Latin -poda, foot) are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling bivalves to which they are not closely related. Paleobiologists estimate that 99 percent of all documented brachiopod species are both fossils and extinct. [more]

Bryozoa

Bryozoans, also known as ectoprocts, are tiny colonial that generally build stony skeletons of calcium carbonate, superficially similar to coral (although some species lack any calcification in the colony and instead have a mucilaginous structure). Members of the phylum Bryozoa are known as "moss animals" or "moss animacules" (which is the literal translation of the Greek term ß?????a, "bryózoa") or as "sea mats". They generally prefer warm, tropical waters, but are known to occur worldwide. There are about 8,000 living species, with several times that number of fossil forms known. [more]

Chaetognatha

Chaetognatha, meaning hair-jaws, is a of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering. [more]

Chordata

Chordates ( Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. The phylum Chordata consists of three subphyla: Urochordata, represented by tunicates; Cephalochordata, represented by lancelets; and Craniata, which includes Vertebrata. The Hemichordata have been presented as a fourth chordate subphylum, but they are now usually treated as a separate phylum. Urochordate larvae have a notochord and a nerve cord but these are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no brain or specialist sense organs, and a very simple circulatory system. Craniates are the only sub-phylum whose members have skulls. In all craniates except for Hagfish, the dorsal hollow nerve cord has been surrounded with cartilaginous or bony vertebrae and the notochord generally reduced; hence hagfish are not regarded as vertebrates. The chordates and three sister phyla, the Hemichordata, the Echinodermata and the Xenoturbellida, make up the deuterostomes, one of the two superphyla that encompass all fairly complex animals. [more]

Cycliophora

Symbion is the name of a of aquatic animals, less than ½ mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983. [more]

Dicyemida

Echinodermata

Echinoderms (Phylum Echinodermata) are a of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period [more]

Echiura

The Echiura, or spoon worms, are a small group of animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum. However, phylogenetic analyses of DNA sequences place echiurans and pogonophorans within the Annelida. The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (called the Pennsylvanian in North America). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian. [more]

Entoprocta

Entoprocta (Gr. e?t??, entos inside + p???t??, proktos anus) is a of small aquatic animals, ranging in size from 0.5 mm to 5.0 mm. They have a lophophore, and as their name suggests, are distinguished from other lophophorates by the position of the anus inside the ring of cilia rather than outside. Other names include goblet worm and kamptozoan. [more]

Gastrotricha

The gastrotrichs (from ?aste?, gaster "stomach" and ????, thrix "hair") are a phylum of microscopic (0.06-3.0 mm) animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles. The Gastrotrich has the shortest life span of all animals, living for just 3 days. [more]

Gnathostomulida

Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine . Most measure between 0.5 and 1 mm long. Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell. They have no body cavity, and no circulatory or respiratory system. Each gnathostomulid is simultaneously hermaphrodite, possessing an ovary and a testis. They are characterized by a specialized, muscular jaw, which they use to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera. [more]

Hemichordata

Hemichordata is a 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. 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. [more]

Kinorhyncha

Kinorhyncha (Gr. ?????, kineo 'move' + ??????, rhynchos 'snout') is a of small (1 mm or less) marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. [more]

Loricifera

Mollusca

Molluscs are animals belonging to the Mollusca. There are around 93,000 recognized extant species, making it the largest marine phylum with about 23% of all named marine organisms. Representatives of the phylum live in a huge range of habitats including marine, freshwater, and terrestrial environments. Molluscs are a highly diverse group, in size, in anatomical structure, in behaviour and in habitat. [more]

Nematoda

The "roundworms" or "nematodes" ( Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic. It has been estimated that the total number of described and undescribed roundworms might be more than 500,000. Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube at both ends. [more]

Nematomorpha

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a of parasitic animals that are morphologically and ecologically similar to nematode worms, hence the name. They range in size from 1cm to 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, Orthoptera and crustaceans. About 326 species are known and a conservative estimate suggests that there may be about 2000 species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots. [more]

Nemertea

Nemertea is a of invertebrate animals also known as ribbon worms or proboscis worms. Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial forms; they are found in all marine habits, and throughout the world's oceans. Nemerteans are named for Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini and Nemertinea. Libbie Hyman named them Rhynchocoela, a name used primarily in North America but gradually abandoned since the 1980s. [more]

Onychophora

The velvet worms (Onychophora — literally "claw bearers") form a within the Ecdysozoa and can be simply described as "worms with legs". Most common in the Southern Hemisphere, they prey on smaller animals such as insects, which they catch by squirting a sticky slime. In modern zoology they are particularly renowned for their curious mating behaviour and for bearing live young. They are becoming increasingly popular in the 'exotic pets' trade, due to their bizarre appearance and eating habits. The Lobopodia, possible ancestors of velvet worms from the Cambrian period, are of great interest in paleontology. [more]

Orthonectida

Platyhelminthes

The flatworms, known in scientific literature as Platyhelminthes (from the p?at?, platy, meaning "flat" and ??µ??? (root: ??µ???-), helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. [more]

Priapulida

Priapulida (priapulid worms or penis worms, from Gr. p???p??, priapos '' + Lat. -ul-, diminutive) are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Besides arthropods and velvet worms, it is only among Priapulida that we can find members of the Ecdysozoa which are relatively large in size. They were likely major predators of the Cambrian period. There are 16 known species of Priapulid worms. [more]

Rotifera

The rotifers make up a of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703. Most rotifers are around 0.1–0.5 mm long (although their size can range from 50µm to over 2 millimeters), and are common in freshwater environments throughout the world with a few saltwater species. Some rotifers are free swimming and truly planktonic, others move by inchworming along the substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers play an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil. [more]

Sipuncula

The Sipuncula or Sipunculida, sipunculid worms or peanut worms, are a containing 144-320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Sipunculid worm jelly (???) is a delicacy in the town of Xiamen in Fujian province of China. [more]

Tardigrada

Tardigrades (commonly known as water bears) form the Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1777. The name water bear comes from the way they walk, reminiscent of a bear's gait. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm. [more]

Vetulicolia

[more]

At least 3 species and subspecies belong to the Phylum Vetulicolia.

More info about the Phylum Vetulicolia may be found here.

References

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  1. ^ Knoll, Andrew H. and Sean B. Carroll. (1999) Early Animal Evolution: Emerging Views from Comparative Biology and Geology. Science. 25 June 1999: Vol. 284. no. 5423, pp. 2129–2137. Found at [1] — URL retrieved November 15, 2006
  2. ^ Balavoine, Guillaume, & Adoutte, Andre. 2003. The segmented Urbilateria: A testable scenario. Integrative & Comparative Biology 43: 137–147. Found at [2] — URL retrieved November 15, 2006
  3. ^ For refs see Ediacara biota
  4. ^ For refs see Vernanimalcula
  5. ^ For refs see Fossil embryos
  6. ^ The Invertebrate Animals
  7. ^ *Helfenbein, Kevin G., H. Matthew Fourcade, Rohit G. Vanjani, and Jeffrey L. Boore (2004). The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes. Proceedings of the National Academy of Sciences of the United States of America 101(29), July 20 2004: 10639–10643.
  8. ^ Papillon, Daniel, Yvan Perez, Xavier Caubit, and Yannick Le Parco (2004). Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome. Molecular Biology and Evolution 21(11), November 2004: 2122–2129.
  9. ^ Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Second Edition. Oxford University Press, Oxford.

Footnotes

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  1. ^ Knoll, Andrew H. and Sean B. Carroll. (1999) Early Animal Evolution: Emerging Views from Comparative Biology and Geology. Science. 25 June 1999: Vol. 284. no. 5423, pp. 2129 - 2137. Found at [1] - URL retrieved November 15, 2006
  2. ^ Balavoine, Guillaume, & Adoutte, Andre. 2003. The segmented Urbilateria: A testable scenario. Integrative & Comparative Biology 43: 137-147. Found at [2] - URL r etrieved November 15, 2006
  3. ^ For refs see Ediacara biota
  4. ^ For refs see Vernanimalcula
  5. ^ For refs see Fossil embryos
  6. ^ The Invertebrate Animals
  7. ^ Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Second Edition. Oxford University Press, Oxford.

Sources

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Last Revised: September 22, 2009
2009/09/22 05:49:09