The mesozoans are tiny parasitic animalcules found in certain marine invertebrates. There are two orders, the Dicyemida and the Orthonectida, whose simple multicellular structure has provoked much speculation about their place in any systematic classification of the animal kingdom.
In a way still unknown, the worm-like dicyemids infect the kidneys of young squids and octopuses. Typically, each is up to 7 mm. long and consists usually of fewer than 25 elongated cells, the so-called axial cells. Within these, other cells are produced, each of which develops into a new individual before breaking out. This development from unfertilized `agametes’, formed without meiosis, never occurs in the Metazoa. The new individuals remain in the kidney of the host and give rise to a further generation in the same way. When the host reaches sexual maturity, this type of reproduction ceases and the dicyemid differs slightly in appearance and reproduces in a different manner: clusters of cells are produced inside the axial cell, the outer cells in the cluster separating off and dividing to form free-swimming ciliated larvae. These leave the kidney of the host, but whether they infect an intermediate host is not known.
At one stage of their cycle, the orthonectids exist as multinucleate amoeboid plasmodia in the tissues of their hosts, which may be flatworms, nemertine worms, annelids, brittle stars, or bivalve molluscs. The plasmodia reproduce by fragmentation for a time, but eventually, like the dicyemids, they produce agametes, which give rise to distinct sexual forms (male, female, or hermaphrodite) with an outer ciliated cell layer, as in dicyemids, but marked off in rings. An inner mass of cells replaces the single axial cell. Generally, the males are about 0.1 mm. long, and the females two or three times this length. As adults, they leave the host and enter the sea, where they copulate. The resulting fertilized eggs develop into ciliated larvae, which then escape from the parent and infect new hosts. At this point, the larvae lose their outer layer of cells, and the inner cells scatter, each giving rise to a new plasmodium to complete the cycle.
In lacking well-defined ectoderm and endoderm, the mesozoans are clearly outside the main body of multicellular organisms, the Metazoa. They could represent a stage in the evolution of that group from unicellular organisms, in which case the first mesozoans can hardly have inhabited the bodies of invertebrates, as they do now. It has also been suggested that they owe their simple structure and complex life-cycles to their parasitic way of life since these are two common features of parasites. This would make them degenerate forms, and one hypothesis is that they could have evolved from flatworms, although there is little to support this. It is, however, more probable that, like the sponges, they represent an independent line of evolution.
The medusa—represented here by Olindias phosphorica—is a free-swimming coelenterate form with a characteristic radial canal system and bell formation. It is essentially a free-living organism as opposed to the polyp, the other highly contrasting mode of life among the coelenterates.