These are very small animals, ranging from 0.5mm - 7 mm in length and typically just 20-30 cells in all. Mesozoans are of worm-like construction, with a ciliated epidermis. They have an entirely parasitic lifestyle, living in the bodies of a variety of different marine invertebrates. There are two principal phyla, the Rhombozoa and the Orthonectidea, both having complex, but poorly understood life histories. Rhombozoans have sexual and asexual stages to their life cycle, and parasitise benthic cephalopods. Orthonectideans are similarly parasitic, tending to target molluscs and polychaetes hosts. The asexual stage of the life cycle is known as the plasmodium.