Within the Phylum Porifera, glass sponges (Class Hexactinellida) are an anomaly. As a group, they are almost completely restricted to the deep sea, they can fuse their skeletons into rigid scaffoldings of glass, and most remarkably, they are the only animal group whose whole body organization is syncytial rather than cellular. Hexactinellid larvae are cellular until the 32-cell stage, when cells begin to divide unevenly and fuse together. The sponges are then syncytial for the rest of their lives. While the formation of the larvae is well described, how adult hexactinellids grow and produce new syncytial tissue is unknown. We collected the glass sponge Aphrocallistes vastus from 40 meters depth in Saanich Inlet, BC and exposed portions of the body tissues to EdU, a thymidine analog, for up to 5 days. Very few new nuclei labeled in the main body of A. vastus. However, growing tissues at the lip of A. vastus revealed a surprising finding: EdU-labeled ‘cells’ formed a band in the lip of the osculum. We have since observed new skeletal growth also occurring in that region using the ratiometric dye PDMPO. Using fluorescence, scanning, and transmission electron microscopy, we find that the labeled cells more closely resemble demosponge choanocytes than the anucleate collar bodies that populate chambers of the adult glass sponge. These findings prompt a completely new view of glass sponge syncytia: that chambers are first formed with similar structure to that of cellular sponges, but then grow in size, recessing the nuclei below collar bodies as they form part of the main body of the sponge. Adult glass sponges thus produce syncytial tissue from a cellular foundation.