This paper presents the morphology of one of the most complex and species-rich families of polychaetes. The body development of spionids and the relationship between body shape and lifestyle are studied. The evolutionary trend of each of the morphological structures of spionids is analyzed, and several hypotheses are formulated. One of these is that branchiae without pinnae are the plesiomorphic condition, as they are found throughout the family, while branchiae with pinnae are the derived or apomorphic condition. The evolutionary trend of branchiae fusion with respect to notopodal lamellae is also analyzed, and three hypotheses are proposed.
