SYNOPSIS. Protostomes and deuterostomes can be characterized by two completely separate sets of characters. Protostome characters are downstream- collecting ciliary bands with compound cilia on multiciliate cells and central nervous system with apical brain and ventral paired or fused cords; spiral cleavage is only found in the protostome group Spiralia. Deuterostome characters are upstream-collecting ciliary systems with separate cilia on monociliate cells, dorsal central nervous system and loss of the larval apical organ in the adults, and archimery with prosome, mesosome and metasome. Only the ectoproct bryozoans lack all the justmentioned characters; they are referred to the protostomes on basis of their metamorphosis which resembles that of entoprocts. The two bilateral groups have probably evolved independently from a radial, gastraea-like ancestor