KINGDOM ANIMALIA
| SYSTEMATIC BIOLOGY |
KINGDOM ANIMALIA |
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| PHYLUM ORTHONECTOZOA | |||||
INTRODUCTION TO THE ORTHONECTOZOA
Orthonectozoa (OR-tho-nek-to-ZO-a) is made of three Greek roots meaning "straight swimming animals" [straight -ortho (ορθό); swimming -nekto (νηκτικο); animal -zoo (ζώο)].
The orthonectids, like the rhombozoans, are among the simplest of the bilaterians in form. As mature organisms, they are little more than a jacket of ciliated cells that enclose a gonad; however, their structural simplicity likely is a consequence of their parasitic lifestyle. In their asexual stages, the dominant forms, all are tissue parasites of an eclectic mix of invertebrates (echinoderms, nemertenians, turbellarian flatworms, polychaetes, and clams; Margulis and Schwartz 1998). The sexual stages are free-swimming (Figure A). Aside from the gametes and zygotes, they form three distinct structural forms in their lifecycle, one of which is a syncitial plasmodium, which can become large enough in some taxa to castrate or do other great harm to their hosts. Once they were grouped together with the Rhombozoa as the Mesozoa, which, as the name implied, were considered transitional organisms at the base of the metazoans (Storer and Usinger 1965). Now, both the rhombozoans and orthonectids are viewed as problematic taxa with no clear affinities (Margulis and Schwartz, 1998; Brusca and Brusca, 2003; and Tudge, 2000). Brusca and Brusca (2003) suggest that the rhombozoans and orthonectids may only appear similar through convergence. That is, highly simplified parasitic animals at the lower end of structural complexity would likely converge on similar body plans. So, their taxonomic association is necessarily suspect. The suggestion by Margulis and Schwartz (1998) that they are related to other acoelomates (particularly the flatworms) is equally suspect.
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Figure A. A diagram of an adult male (left) and female (right) of Rhopalura. |
| Image taken from: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=MESO003B&File_type=GIF |
SYNOPTIC DESCRIPTION OF THE PHYLUM ORTHONECTOZOA
| The following information came from Margulis and Schwartz (1998), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Storer and Usinger (1965), and Tudge (2000). |
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I. SYNONYMS: orthonectids, mesozoans. II. NUMBER: >40 species known. III. PHYLUM CHARACTERISTICS:
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HIERARCHICAL CLASSIFICATION OF THE ORTHONECTOZOA
| Taxonomy of the Phylum follows the system of Brusca and Brusca (2003). However, I have changed their name of the phylum from Orthonectida (Brusca and Brusca, 2003) to Orthonectozoa to decrease confusion with standard class names. |
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This page is maintained by Jack R. Holt. Last modified: 03/14/08
