SYSTEMATIC BIOLOGY					KINGDOM ANIMALIA
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY	TAXA OF LIFE
PHYLUM RHOMBOZOA

INTRODUCTION TO THE RHOMBOZOA

Rhombozoa (rom-bo-ZO-a) is made of two Greek roots that mean a rhomboid animal [rhomboid -rhomboeides (ρομβοειδές); animal -zoo(ζώο)].

The rhombozoans are among the simplest of the bilaterians in form with twenty to thirty somatic cells enclosing a long, cylindrical axial cell with one to several hundred axoblasts (Figure A). However, because they are parasites of cephalopod kidneys, their structural simplicity likely is a consequence of their parasitic lifestyle.  Their lifecycles are poorly known, but seem to be elaborate with at least four different structural (and functional) stages.  Once they were grouped together with the Orthonectida 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.

SYNOPTIC DESCRIPTION OF THE RHOMBOZOA

HIERARCHICAL CLASSIFICATION OF THE RHOMBOZOA

This page is maintained by Jack R. Holt.  Last modified: 03/14/08