METAMORPHOSIS OF CALCAREOUS SPONGES .1. ULTRASTRUCTURE OF FREE-SWIMMING LARVAE

Free-swimming larvae of two calcareous sponges were studied by electron microscopy. The larvae are composed of four kinds of cells, namely flagellated cells, granular cells, four cruciform cells, and several yolk-containing cells. In the anterior hemisphere of the larvae, the columnar flagellated cells are arranged in a single layer. Their nucleus and Golgi apparatus are located in close proximity to the flagellar rootlets. There are granular cells in the posterior hemisphere of the larvae. They have a nucleus with a nucleolus, large phagosomes, well-developed Golgi apparatus, and numerous RER cisternae. There is a cruciform cell in each quadrant of the larvae. From its characteristic arrangement of organelles, it is suggested but not concluded that the cruciform cells participate in photoreception. The yolk-containing cells are, most probably, nutritive cells derived from the mother sponge. The roles of these four kinds of cells in habitat selection and metamorphosis are discussed.