Sponge larvae possess cilia with unusual terminal expansions which are curled or biconcave in shape externally, as seen by scanning electron microscopy. Thin sections, passing through the point where the ciliary shaft enters the expanded area reveal the ciliary axoneme to be surrounded by many membranous folds, some of which are vesicular. The 'club footed' cilia occur in disparate groups of Demospongiae and most frequently, all larval cilia are of this type. There is no immediately obvious correlation between the type of movement displayed by the larvae and the occurrence of terminally expanded cilia. © 1977.
Bergquist, P. R., Green, C. R., Sinclair, M. E., & Roberts, H. S. (1977). The morphology of cilia in sponge larvae. Tissue and Cell, 9(1), 179–184. https://doi.org/10.1016/0040-8166(77)90058-1