Developmental Patterns - Metazoa
The metazoans or Animalia can be divided into two groups on the basis of body symmetry. The bilateral metazoans can be divided into two great assemblages: the Protostomia and the Deuterostomia.
Platyhelminthes, Molluscs, Annelida, Arthropods and a number of minor phyla are classified as Protostomes while Echinodennata, Chordata and at least two minor phyla are included in the deuterostomes. The features used to place animals in these groups are largely developmental.