Evolution of Metazoa

The sponges, coming under phylum Porifera are the closest to Protista, and can perhaps be regarded even as a colony of protists rather than being multicellular. No other group has evolved from them. The coelenterates consisting of the Cnidaria and the Ctenophora, which are both diploblastic and primarily radially symmetrical, can be regarded as truly the most primitive of the Metazoa. They have evolved from the ancestral planuloid metazoans as an offshoot:

Figure: Phylogeny of the animal kingdom

The Platyhelminthes (flatworms) have also evolved from planuloid ancestor. They do not have a coelomic cavity (acoelomates) but the mesoderm is cellular, they are triploblastic and are bilaterally symmetrical. This group and all higher Metazoa can be grouped as Grade Bilateria.

The pseudocoelomates phyla which include the nematodes and the rotifers are supposed to have evolved as an offshoot from the flatworms.

The Eucoelomata constitute the remainder of the Metazoa. The acoeloid flatworm like ancestors have given rise to two main stocks:

1. The molluscan - annelidan, arthropodan stock with schizocoelous coelom.
2. The echinoderm, hemichordate, chordate stock with enterocoelic coelom. The coelom serves as skeleton in the more primitive of the coelomates. Such as the polycheate worms, earthworms-etc. In arthropods and molluscs the coelom becomes reduced to the point of being represented by the cavity of the gonads. The other space in arthropod body is only a haemocael which is a mere space in the tissue filled with blood.