Sponge development is unique among metazoans, and many controversies remain surrounding the cells' fates and lineages in this process. In the series of our reports, the metamorphosis of the demosponge Haliclona sp. is elucidated with clear electron micrographs using an improved method for electron microscopy. In this report the fine structure of the free-swimming parenchymella larva is described. The parenchymella larva of this sponge is three-layered, with a flagellated epithelium, an intermediate layer, and a central cell mass. The flagellated columnar epithelium is a locomotive organ and covers the larva except at the posterior pole, which is covered with a layer of archeocytes instead. A few vesicular cells are scattered in the flagellated epithelium. In the intermediate layer collencytes with many small vesicles predominate. In the central cell mass there are archeocytes with a conspicuous nucleolus, scleroblasts with a monaxon spicule, gray cells loaded with yolk-like granules, and granulous cells with a peculiar morphology. Thus the parenchymella larva of Haliclona sp. consists of seven kinds of cells in all. On the basis of the detailed morphology of these larval cells, their functions in the swimming period and metamorphosis are discussed.
