Traditionally, lithistid sponges have been combined within one order linked by the common possession of an interlocking siliceous desma skeleton. However, molecular data, morphology, and paleontology confirm recent hypothesis of polyphyly for this enigmatic group of sponges. Parsimony and neighbor-joining analysis of partial 18S rDNA sequences derived from amplified genomic rDNA of lithistids and other demosponges suggest that tetracladinid and dicranocladinid lithistids are monophyletic and closely related to choristid demosponges. Rhizomorinid families, in contrast, are indicated as being polyphyletic, with a diversity of points of origin within the tetractinomorph and the ceractinomorph poriferan subclasses. These arrangements are discussed in light of several hypotheses of demosponge evolution.