DESCRIPTION OF THE PHYLUM PLACOZOA (GRELL 1971)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>PARAZOA>PLACOZOA |
PHYLUM PLACOZOA LINKS
| Placozoa (plak-o-ZO-a) is a combination of two Greek roots that mean flat animal [plax, a flat tablet or table (plax); and zoa, an animal (ζώο)]. This is a reference to the flattened appearance of the animal. |
| INTRODUCTION TO THE PACOZOA Defined by Grell (1971), the Placozoa is a small group of simple organisms with very little structural complexity, a grade that Brusca and Brusca (2003) call the ‘Mesozoan Grade of Complexity”. The upper and lower surfaces of this irregular and flattened animal are made of ciliated epithelial (text with tooltip) Epithelium is a tissue that covers (or lines) a structure. Ciliated epithelium is made of epithelial cells that may have one or more flagella (=cilia) inserted into it. It is the chief means of locomotion for many invertebrates. cells that envelop a loosely-organized mesenchymal layer (See Figures 1 and 2). As an animal, it is very odd. The mitochondria have tubular cristae (text with tooltip) Mitochondrial cristae that form as extended saccate structures or tubes are called tubular cristae. , and the extracellular matrix has no collagen (text with tooltip) Collagen is an animal protein that makes up extracellular skeletal materials. (Adl et al. 2005). According to Barnes (1980) members of the group, particularly Trichoplax, resemble the planula larva (text with tooltip) The planula is a larval form of certain Cnidaria. They are bilaterally symmetrical, covered with ciliated epithelium, and free-swimming. of the Radiata. Thus, the group may represent either an ancestral complexity commensurate with their appearance or a derived, neotenic (text with tooltip) Neoteny is the situation in which an immature form becomes sexually mature in the developmental history. form. Either way, the animal has such a reduced genome that it approximates that of a bacterium (Brusca and Brusca 2003). Adl et al. (2005) treat the Placozoa as a group equivalent to the Porifera, Mesozoa, and the Animalia. Borchiellini et al. (2001; see Figure 3 below) in an analysis based on 18S rDNA sequences shows Trichoplax emerging at the base of a clade that includes the Cnidarians. If this is the case, then the concept of Parazoa can only be thought of as a grade rather than a clade. It might also confirm that Trichoplax is a neotenic cnidarian. |
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| FIGURE 1. Labeled drawing of Trichoplax, the single genus in this phylum. Image from http://www.ldeo.columbia.edu/edu/dees/ees/life/slides/phyla/placozoa.html | FIGURE 2. Photomicrograph of Trichoplax, the single genus in this phylum. Image by Hunadam, Wikimedia Commons |
FIGURE 3. A figure from Borchiellini et al. (2001) based on 18S rDNA sequences. The Placozoa (Trichoplax) emerge in a clade that is basal to the Cnidarians and not associated with the sponge groups.
| LITERATURE CITED Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5): 399-451. 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. Bergquist, P.R. 2001. The Porifera. In: Anderson, D.T., ed. Invertebrate Zoology. Oxford University Press. Oxford, UK. pp. 11-27. Borchiellini, C., M. Manuel, E. Alivon, N. Boury-Esnault, J. Vacelet, and Y. Le Parco. 2001. Sponge paraphyly and the origin of Metazoa. J. Evol. Biol. 14: 171-179. Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. 2nd Edition. Sinauer Associates. Sunderland, MA. Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. Grell, K. G. 1971. Trichoplax adhaerens F. E. Schulze und die Entstehung der Metazoen. Natwiss. Rundsch. 24: 160–161. 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. 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. Sperling, E.A., D. Pisani, and K.J. Peterson. 2007. Poriferan paraphyly and its implications for Precambrian palaeobiology. In: Vickers-Rich, P. and P. Komarower, eds. The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications. 286: 355-368. 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. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 08/28/2016 |
