Mesozoa are small vermiform (form of a worm) animals formed by a few cells (among 20 and 30 cells, depending on the species) arranged in two layers that are not equivalent to the layers of metazoans. All mesozoans live as parasites of marine invertebrates and their cycle of life is very complex and insufficiently known. Its locomotion is carried out by cilia. Some time ago, mesozoa were considered as a link between protozoan and metazoan, because they do not possess true tissues and their structure is very simple.
The Phylum includes two classes:
- Class Rhombozoa- Parasites of the renal bags of cephalopods, like octopi, sepias and squids. Two orders:
- Class Orthonectida- Parasites of Ophiuroidea, bivalve mollusks, polychaeta and nemertean. They are formed by a layer of ciliated cells that contains a mass of gametes. Example of this class: Rhopalura ophiocomae.
Most of biologists are in agreement that these minuscule animals are primitive or simplified Platyhelminthes, although some biologists consider that mesozoans are a phase in the course of the evolution from protozoan to metazoan. Some zoologists claim that mesozoans are descendants of ciliated protozoan.
As you can see, these tiny animals are a big case... he, he, he... Personally, I consider they are simplified Platyhelminthes; precisely, because of their parasitic way of life.