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Chaetognatha is a of predatory marine worms that are a major component of worldwide. About 20% of the known species are benthic and can attach to 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 (Bone et al 1991).
Chaetognaths are transparent or translucent and are covered by a . They have fins and a pair of hooked, , grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. They have a distinct head, trunk and tail. All species are , carrying both and . Some species are known to use the neurotoxin to subdue prey (Thuesen 1991).
They have some developmental similarities to . Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory, circulatory, or excretory systems. Materials are moved about the body cavity by . Waste materials are simply excreted through the skin and anus. Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering (Jordan 1992). At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins (Haddock & Case 2004).
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Chaetognaths are traditionally classed as by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification (, per Margulis & Schwartz). Molecular phylogeneticists, however, consider them to be . Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification (, per Cavalier-Smith). The similarities between chaetognaths and nematodes mentioned above may support the protostome hypothesis - in fact, chaetognaths are sometimes regarded as basal (Matus et al. 2006). Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny (Marletaz et al. 2006). This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they may have retained deuterostome characters inherited from early ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian (Papillon et al. 2004).
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Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described (Vannier et al. 2007). Chaetognaths appear to have originated in the period. Complete body fossils have been formally described from the of Yunnan, China (Eognathacantha ercainella Chen & Huang 2002 and Protosagitta spinosa Hu 2005) and the of British Columbia (Oesia disjuncta Walcott: Szaniawski 2005). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the . The conodonts themselves, however, are thought to be related to the . It is now thought that elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then (Szaniawski 2002). 14
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Chaetognatha [Chaetognathi] |--Paucijaculum samamithion Schram 1973 S02 |--Spadella cephaloptera WT06 |--Protohertzina Missarzhevsky 1973 S02 |--Eukrohnia hamata B26 |--Pterosagitta draco B26 |--Eognathacantha Chen & Huang 2002 CH02 | `--*E. ercainella Chen & Huang 2002 CH02 |--Paraspadella [Phragmophora] WT06 | `--P. gotoi WT06 |--Phakelodus S02 | |--P. elongatus (Zhang in An et al. 1983) S02 | |--P. savitzkyi S02 | `--P. tenuis (Müller 1959) [=Prooneotodus tenuis] S02 |--Coelocerodontus Ethington 1959 S02 [incl. Hertzina Müller 1959 DJ71] | |--*C. trigonius Ethington 1959 | |--C. biconvexus Bultynck 1970 WSQ86 | |--C. burkei Druce & Jones 1971 [incl. Proconodontus mulleri] DJ71 | |--C. primitivus (Müller 1959) [=Furnishina primitiva] DJ71 | |--C. rotundatus Druce & Jones 1971 DJ71 | `--C. tricarinatus (Nogami 1967) [=Hertzina tricarinata] DJ71 |--Sagitta Quoy & Gaimard 1827 [Aphragmophora] S02 | |--S. crassa KW95 | |--S. elegans GD00 | |--S. enflata B26 | |--S. hexaptera B26 | |--S. lyra B26 | |--S. maxima Conant 1896 S02 | |--S. scrippsae B70 | |--S. serratodentata B26 | |--S. tasmanica B70 | `--S. zetesios B70 |--Amiskwia Walcott 1911 S02, G89 | `--A. sagittiformis Walcott 1911 S02 |--Flacisagitta enflata DH08 `--Aidanosagitta Tokioka & Pathansali 1963 KS03 |--*A. neglecta (Aida 1897) KS03 |--A. acus Kasatkina & Selivanova 2003 KS03 |--A. bedfordii (Doncaster 1902) KS03 |--A. bella Kasatkina & Selivanova 2003 KS03 |--A. coreana (Molchanov 1907) KS03 |--A. crassa (Tokioka 1938) KS03 | |--A. c. crassa KS03 | `--A. c. naikaiensis (Tokioka 1939) KS03 |--A. delicata (Tokioka 1939) KS03 |--A. demipenna (Tokioka & Pathansali 1963) KS03 |--A. erythraea (Casanova 1985) KS03 |--A. firmula Kassatkina 1971 KS03 |--A. golicovi Kasatkina 1971 KS03 |--A. lacuna (Tokioka 1942) KS03 |--A. lepida Kasatkina & Selivanova 2003 KS03 |--A. macilenta Kassatkina 1971 KS03 |--A. modica (Kassatkina 1971) KS03 |--A. murex Kasatkina & Selivanova 2003 KS03 |--A. oceanica (Gray 1930) KS03 |--A. parva (Oye 1918) KS03 |--A. pilum Kasatkina & Selivanova 2003 KS03 |--A. regularis (Aida 1897) KS03 |--A. scarlatovi Kassatkina 1971 [=A. scarlatoi] KS03 |--A. septata (Doncaster 1902) KS03 |--A. tumida (Tokioka 1939) KS03 `--A. venusta Kasatkina & Selivanova 2003 KS03
* Type species of generic name indicated
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Taxonomy 23:15, 7 January 2011 (UTC)