Whatb is the criteria for dividing kingdom animalia into mesozoa and metazoa?
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Phylum Mesozoa
Phylum Mesozoa
The name Mesozoa (mes-o-zo´a) (Gr. mesos, in the middle, + zoon, animal) was coined by an early investigator (van Beneden, 1876) who believed that the group was a “missing link” between protozoa and metazoa. These minute, ciliated, wormlike animals represent an extremely simple level of organization. All mesozoans live as parasites in marine invertebrates, and the majority of them are only 0.5 to 7 mm in length. Most are composed of only 20 to 30 cells arranged basically in two layers. The layers are not homologous to the germ layers of higher metazoans.
The two classes of mesozoans, Rhombozoa and Orthonectida, differ so much from each other that some authorities place them in separate phyla.
Rhombozoans (Gr. rhombos, a spinning top, + zoon, animal) live in kidneys of benthic cephalopods (bottom- dwelling octopuses, cuttlefishes, and squids). Adults, called vermiforms (or nematogens), are long and slender (Figure 12-1). Their inner, reproductive cells give rise to vermiform larvae that grow and then reproduce. When a population becomes crowded, reproductive cells of some adults develop into gonadlike structures producing male and female gametes. Zygotes grow into minute (0.04 mm) ciliated infusoriform larvae (Figure 12-1B), quite unlike the parent. These larvae are shed with host urine into the seawater. The next part of the life cycle is unknown because infusoriform larvae are not immediately infective to a new host.