Classification of Kingdom Animalia
Animal kingdom is divided into two sub-kingdoms, the Parazoa and Eumetazoa based on their organisation.
1. Parazoa: These include the multicellular sponges and their cells are loosely aggregated and do not form tissues or organs.
2. Eumetazoa: These include multicellular animals with well defined tissues, which are organised as organs and organ systems. Eumetazoans includes two taxonomic levels called grades. They include Radiata and Bilateria.
Among the eumetazoa, a few animals have an organisation of two layers of cells, the outer ectoderm and inner endoderm, separated by a jelly like mesoglea. They are radially symmetrical and are diploblastic.
Examples: Cnidarians (sea anemone, jelly fish) and Ctenophores (comb jellies).
The eumetazoans other than Radiata, show organ level of organisation and are bilaterally symmetrical and triploblastic. The grade Bilateria includes two taxonomic levels called Division.
Protostomia includes the eumetazoans in which the embryonic blastopore develops into mouth. This division includes three subdivisions namely acoelomata, pseudocoelomata and schizocoelomata.
Eumetazoans in which anus is formed from or near the blastopore and the mouth is formed away from the blastopore. It -includes only one subdivision Enterocoelomata. They have a true coelom called enterocoel, formed from the archenteron.