“…They are one of the most phylogenetically divergent groups of animals (King and Rokas, 2017;Simion et al, 2017), their anatomy is fundamentally different from other animals (Leys and Hill, 2012), and there are long-standing questions about the structure and homology of their tissues compared to epithelia in other animals (Leys et al, 2009;Tyler, 2003). It has been argued that sponge cell adhesion relies primarily upon an extracellular proteoglycan complex termed the Aggregation Factor (Bucior and Burger, 2004;Cauldwell et al, 1973;Grice et al, 2017;Haseley et al, 2001;Henkart et al, 1973;Humphreys, 1963;Vilanova et al, 2016). Antibodies raised against the Aggregation Factor have been reported to block reaggregation of dissociated cells (Schütze et al, 2001), and purified Aggregation Factor can mediate adhesion between beads in cell-free assays (Jarchow and Burger, 1998).…”