On the basis of embryonic development, metazoans are divided into Protostomia and Deuterostomia. Protostomes are primitive invertebrates while deuterostomes include echinoderms and chordates. This division helps in understanding relationships of different groups of animals.
PROTOSTOMES
Protostomes include flat worms, annelids, arthropods, molluscs and some minor phyla.
During embryonic development blastopore forms the mouth on the anterior end and anus appears later to complete the alimentary canal.
Nerve cord is ventral in protostomes.
Fate of blastomeres is determined very early during holoblastic cleavage. This is called determinate cleavage, which means blastomeres are destined to form a particular organ in very early stage of cleavage.
Cleavage is spiral in protostomes, i.e. axis of cleavage plane is oblique, so that blastomeres have a spiral arrangement in which one tier of cells alternates with the next tier of cells. This is also called mosaic cleavage.
Mesodermal tissue is formed by the division of a single blastomere 4d cell.
Origin of coelom takes place by schizocoely by splitting of the mesodermal cell mass.
DEUTEROSTOMES
Deuterostomes include echinoderms, chordates, pogonophora, hemichordates and some minor phyla.
Blastopore forms anus during embryonic development and represents the posterior end of body. Mouth is formed later.
Cleavage is indeterminate and if blastomeres are separated at 4 cell stage, each one will develop into a complete individual.
Cleavage is radial, in which the cleavage plane is either parallel or at right angle to the polar axis. Blastomeres are arranged directly above or below one another. This is also called regulative cleavage.
Mesodermal tissue is formed by the outgrowth of endodermal wall of the archenteron.
Coelom is formed by enterocoely or outpouching of the archenteron.
Nerve cord is dorsal in deuterostomes.