|Relatively few cells deposited
in only two layers
||Tissues or organs
|Reproduction includes self-fertilising hermaphrodites, sexual and asexual.
||Digestive and excretory system
This small phylum contains only
fifty species in two Classes, the Rhombozoa and Orthonecta.
The Rhomboza are parasites of Cephalopod Molluscs. They range in size from 0.5 - 7.5 mm, and
have only 20 - 30 ciliated cells around one single axial cell. Their life-cycle
is not completely known.
The Orthonecta are parasites of marine
invertebrates. They are free-living in the sexual stage. The germinal cells
invade their host, then multiply to form a multinucleate plasmodium that can
reproduce asexually producing other plasmodia. When single-sex cells are
produced they leave the host to reproduce sexually.
It is not known whether the
two Classes are truly related, nor has their taxonomic position been settled,
as some authorities believe they are more closely related to the ciliated
Rhopalura sp., right, is a parasite of Platyhelminthes, Mollusca and Annelida. Its body is simply a layer of epithelial cells, many with cilia, surrounding a group of sex cells.