NIELSEN, C.; Zoological Museum, Copenhagen, Denmark: Trochophora larvae and animal phylogeny

The trochophore has been a central concept in speculations on metazoan evolution for more than a century. Hatschek summarized the more or less scattered observations on ciliated marine larvae over several decades and of his own observations, especially of the larvae of Polygordius and Echiurus. He created the name trochophora for a larval type which he considered characteristic of annelids and molluscs and in more derived forms also of flatworms, bryozoans, phoronids, and echinoderms and discussed a number of features which he considered characteristic of this larval type. He also speculated that the ancestor of these phyla could have resembled the trochophore, as represented by the rotifer Trochosphaera. Later studies of cell lineage, both classical and modern, have contributed importantly to our views on homology of various larval structures, and modern techniques, such as TEM, SEM and molecular methods are adding a wealth of new and important information about structure and development of these larvae. Here, I will try to summarize our knowledge of the development and morphology of ciliated larva with emphasis on the information that cell-lineage studies can give on the homology of various larval organs. This should lead to a more clear definition of the trochophora concept, and a more well-founded opinion on which larvae should be characterized as trochophores (sometimes in more or less modified forms), and which must be excluded.. Finally, I will discuss how the present knowledge of development and morphology of the trochophora larvae can be used to form hypotheses about the evolution of the Protostomia. Such hypotheses can make predictions about development and morphology which can be tested through new investigations.