The lack of any comprehensive and persuasive theory of vertebrate descent is noted. Recent findings are reviewed which appear to demonstrate that phylogenetic inferences cannot be predicated upon echinoderm-hemichordate larval and chemical resemblances. Similar warnings are presented regarding strict application of the recapitulation and germ-layer theories to phylogenetic problems. Given the removal of these impediments, a homogeneous theory is presented, embracing the coelenterates, echinoderms, annelids, arthropods, mollusks, and chordates. Special emphasis is devoted to possible homologies - between the echinoderms (particularly those with pedicellariae, and the holothurians) and the annelids and arthropods, and between the mollusks and lower chordates. A detailed comparison is made of the structures and processes employed by representatives of both of the latter groups in the mucous-ciliary method of feeding and associated respiratory functions. Their possible phylogenetic significance is discussed. The compatibility of a homogeneous theory with the existing theories is suggested.