Reproduc io is predomi a ly sexual, wi h he embryo passi g hrough a blas ula phase . he co spicuous differe ces be wee pla s a d a imals firs led o a formal divisio of all life i o wo ki gdoms, Pla ae a d A imalia. Whe he world of microorga isms was la er i ves iga ed, some were see o be clearly pla like, wi h walled cells a d chlorophyll-co ai i g bodies ha co duc ed pho osy hesis, whereas o hers resembled a imals i ha hey moved abou (by mea s of flagella or pseudopodia) a d diges ed food. he la er ype, pro ozoa, was ca egorized as a subki gdom of A imalia. Difficul ies arose, however, wi h ma y forms ha showed mixed charac eris ics a d wi h groups i which some orga isms were pla like bu had close rela ives ha were a imal-like. Eve ually, classifica io schemes wi h several ki gdoms were proposed, i which he defi i io s of Pla ae a d A imalia became more res ric ed. Wha co s i u es a a imal, herefore, depe ds o he scheme followed. I he five-ki gdom sys em used i his e cyclopedia , a imals are limi ed o orga isms wi h differe ia ed issues, a d he pro ozoa groups are assig ed o he ki gdom Pro is a. he separa io of he pro ozoa from he higher a imals is o e irely sa isfac ory, however, because classifica io sys ems should reflec evolu io ary rela io ships, a d mul icellular forms are believed o have evolved from pro ozoa a ces ors more ha o ce.