Comparative study of the choanosome of Porifera: 1. The Homoscleromorpha

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Abstract
The choanoderm and pinacoderm of representatives of the 2 families of Homoscleromorpha sponges, the Oscarellidae and Plakinidae, were examined by transmission and scanning electron microscopy. Different fixative procedures show the dramatic influence of fixation conditions on the morphology of choanocytes. These 2 families of sponges have the following morphological features in common: flagellated endopinacocytes with short apical microvilli and basal pseudopods; the presence of a very thin and dense sheet of matrix material which limits the mesohyl. There are only minor differences in the flagellar morphology, granule content and anchoring system of their choanocytes. Two findings are of particular interest: the presence of glycocalyx bridges between the microvilli of the choanocyte collar; and the discovery of a new cell type, the apopylar cell, which has a morphology intermediate between that of pinacocytes and choanocytes. The apopylar cells limit the apopylar opening of the choanocyte chamber and indicate the transition between choanoderm and pinacoderm. [The organisms examined included Oscarella lobularis (Schmidt, 1862), Corticium candelabrum (Schmidt, 1862) and Plakina trilopha (Schulze, 1880).].