DESCRIPTION OF THE PHYLUM GNATHOSTOMULIDA (AX 1956)
| EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> PROTOSTOMATA> SPIRALIA> PLATYZOA> GNATHIFERA> GNATHOSTOMULIDA |
GNATHOSTOMULIDA LINKS
| Gnathostomulida (nath-o-STOM-u-li-dah) is made of two Greek roots meaning the jaw mouths [jaw -gnathos (γνάθος); mouth -stoma (στόμα)]. The reference is to the jaw-like structures in the pharynx. |
| INTRODUCTION TO THE GNATHOSTOMULIDA Gnathostomulids are entirely marine and associated with interstitial and detrital habitats. They resemble gastrotrichs (Figure 1) and move about on ciliated epithelium like gastrotrichs. The primary differences have to do with the digestive tract and the fate of the blastocoel. Rather than retaining a persistant blastocoel as a pseudocoelom, gnathostomulids filled the space with parenchymatous mesoderm. The digestive tract is tubular but degenerate in that there is no anus, the gut ends as a caecum. The gnathostomulids have a muscular pharynx equipped with paired jaws (Figure 2), similar to the trophi of the rotifers. Indeed, the similarity is so striking that Rieger and Tyler (1995) concluded that they were sisters to the Rotifer-Acanthocephala phylum (called Syndermata in this system) in a clade called Gnathifera. We have adopted the sister relationship of Rieger and Tyler (1995; see Figure 3). Witek et al. (2009) provided support for the monophyly of the Gnathifera, but they did not include the Micrognathozoa in the analysis. The gnathostomulids comprise a small group of acoelomates that share characters with pseudocolelomates and eucoelomates. Indeed, Nielsen (2001) claims that these are part of the clade that includes the polychaete worms. Clearly, their incomplete digestive tract with a degenerate anus, absence of excretory structures, etc. all suggest that they are highly modified and reduced from a coelomate form. Giribet et al. (2000) grouped them together with taxa both eucoelomate and pseudocoelomate, in a group they called the Platyzoa. Valentine (2004) placed them in the clade that he called the Paracoelomata, which is approximately equal to the Platyzoa. Later, Giribet et al. (2007) placed the Platyhelminthes in the Platyzoa (again), a view adopted by Edgecombe et al. (2011). |
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| FIGURE 1. A light micrograph of Gnathusomula. Image from http://www.zmuc.dk/InverWeb/Dyr/Limnognathia/phylogeny/Gnathostomulida_UK.htm | FIGURE 2. An SEM micrograph of the “jaws” of Gnathostomula. Image from http://www.zmuc.dk/InverWeb/Dyr/Limnognathia/phylogeny/Gnathostomulida_UK.htm |
 | FIGURE 3. A cladogram showing the sister relationship between the Syndermata and the Gnathostomulida as proposed by Rieger and Tyler (1995). Gn = Gnathifera Pl = Platyzoa P = Protostomata |
| LITERATURE CITED Ax, P. 1956. Die Gnathostomulida, einerastelhafte Wurmgruppe aus dem Meerssand. Abhandl. Akad. Wiss. u. Lit. Mainz, math.-naturwiss. 8: 1-32. 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. Giribet, G., D. L. Distel, M. Polz, W. Sterrer and W. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Systematic Biology 49:539-562. 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. 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. Rieger, R. M. and S. Tyler. 1995. Sister-group relationship of Gnathostomulida and Rotifer-Acanthocephala. Invertebrate Biology. 114(2): 186-188. 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: 02/03/2014 |
