Developmental potential of ciliated cells of ceractinomorph sponge larvae.

The fate of the ciliated epithelium of sponge larvae at metamorphosis has been the subject of debate for many years. In order to resolve the phylogenetic position of the Porifera it must be determined whether these cells migrate inwards to become adult choanocytes or are lost at metamorphosis. We report an approach to this problem by observing metamorphosing larvae using electron microscopy and by assaying the aggregation potential of ciliated and central larval cells fractionated on Percoll gradients. Our results indicate that larval epithelial cells are phagocytosed during metamorphosis and that they do not have aggregation capability. The latter is essential to permit choanocyte chamber formation. Larval archaeocytes do possess aggregation capability. We argue that larval ciliated cells are dispersal effectors only and are terminally differentiated. Thus 'inversion of layers' as claimed by earlier authors does not occur. These results are discussed in relation to the role of aggregation facilitating systems and to the structure of sponge larvae.