DESCRIPTION OF THE PHYLUM CHAETOGNATHA (LEUCKART 1854)
| EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> PROTOSTOMATA> CHAETOGNATHA |
PHYLUM CHAETOGNATHA LINKS
| Chaetognatha (ke-TAG-na-tha) is made of two Greek roots meaning bristle jaws [bristle or mane -chaeto (χαίτη); and jaw -gnathos (γνάθος)]. The reference is to the bristles around the mouth of the arrow worm. |
| Chaetognaths or Arrow Worms are distributed as free-swimming animals from the poles to the tropics where they may be found in the open ocean, coastal waters, and in the benthos. They are called Arrow Worms because they superficially resemble tiny planktonic fish with long thin bodies and obvious posterior fin-like tails (Figure 1). They are clear and almost invisible in the water where they actively hunt or ambush copepods and small fish. Their nervous and sensory are also somewhat complex. Arrow Worms have an anterior and posterior ganglion with other lateral ganglia, which together likely process sensory information from the arrays of bristles and the pair of simple eyes to coordinate capturing of prey. The head is slightly bulbous (Figure 2) and surrounded by spines that may be sensory and also help in the mechanics of handling prey. The mouth also has two rows of teeth to help secure and process food. Brusca and Brusca (2003) and Margulis and Schwartz (1998) place the chaetognaths within the deuterostomes, but Nielsen (2001) does not. In fact, Nielsen (2001) argues that the arrow worms are more closely allied to the rotifers and gnathostomes, which would place them within the protostomes. Other treatments such as Tudge (2000) also consider them to be allied to the protostomes. Giribet et al. (2007) and Dunn et al. (2008) place them at the base of the Protostomata as the sister group to all other protostomes or to the Spiralia, respectively. Valentine (2004) placed the the arrow worms within the Paracoelomata (=Platyzoa) but gave them a position of incertae sedis. Edgecombe et al. (2011) point out that a better understanding of the placement of the Chaetognatha could provide insight into critical morphological and developmental characters that were common to some of the earliest bilaterians. We have followed Edgecombe et al. (2011) in placing them at the base of the Protostomata (Figure 3). |
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| FIGURE 1. Sagitta, a common planktonic chaetognath. It is small (8-12 mm long) and resembles a fish. Image from http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/jpegs/sagit.jpg | FIGURE 2. The anterior end of Sagitta. Note the spines and the large chamber associated with the head. Image from http://www.vims.edu/bio/zooplankton/BATS/Chaetognatha/ |
FIGURE 3. A cladogram that highlights the basal nature of the arrow worms within the Protostomata (P) after Edgecombe et al. (2011). B = Bilateria
| LITERATURE CITED Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia. Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257. Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. Dunn, C.W., A. Hejnol, D.Q. Matus, K. Pang, W.E. Browne, S.A. Smith, E. Seaver, G.W. Rouse, M. Obst, G.D. Edgecombe, M.V. Sørensen, S.H.D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R.M. Kristensen, W.C. Wheeler, M.Q. Martindale, and G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 452: 745-749. Edgecombe, G. D., G. Giribet, C. W. Dunn, A. Hejnol,R. M. Kristensen, R. C. Neves, G. W. Rouse, K. Worsaae, and M. V. Sorensen. 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms Diversity and Evolution. DOI 10.1007/s13127-011-0044-4. Giribet, G., C. W. Dunn, G. D. Edgecombe, and G. W. Rouse. 2007. A modern look at the Animal Tree of Life. Zootaxa. 1668: 61-79. Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis. Leuckart, R. 1853-1854. Zoologische Unterschungen. 3 Hefte. Gießen, Germany. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Meglitsch, P. A. and F. R. Schramm. 1991. Invertebrate Zoology. Oxford University Press, New York, Oxford. Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963. Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. Valentine, J. W. 2004. The Origin of Phyla. University of Chicago Press. Chicago. 614 pp. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/10/2013 |
