Characteristics of Metazoa
The unicellular protozoans are highly versatile and successful organisms that show remarkable organization and division of labour within the confines of the single cell. This diversity is achieved by varying the structure of their organelles at the sub-cellular level. The Metazoa or the multicellular animals have achieved their structural diversity by varying their cells that have become specialized to perform different functions. These cells are normally incapable of independent existence. Let us list out some of the features that characterize metazoans.
1. Members of Metazoa possess a complex multicellular structural organization which may include the presence of tissues, organs and organ systems.
2. In the life history of metazoans, typically a fertilized egg passes through a blastula stage in the course of its early embryonic development before changing into an adult.
3. Since metazoans are multicellular they are relatively larger in size than unicellular protozoans. Naturally, their nutritional requirements are more and they have to search for food. Consequently, locomotion in metazoans is highly developed and for this purpose they have evolved contractile muscular elements and nervous structures.
4. The ability for locomotion has influenced the shape of the metazoan animals which in turn has conferred specific types of symmetries to metazoan groups.
5. Most of the metazoans show differentiation of the anterior end or head (cephalization); associated with cephalisation, there is the centralization of the nervous system in the head region. Although all metazoans share some characteristic features, their body plans differ in symmetry, internal organization, developmental patterns and modes of formation of, body cavity. These differences provide us a means of grouping them or organising them into different phyla. Let s discuss these features one by 'one.