The ancestral cleavage pattern of the clitellates and its phylogenetic deviations

The cleavage pattern of the ancestor of the monophyletic taxon Clitellata is reconstructed. It resembles the complicated cleavage pattern of the glossiphoniid leeches in nearly every detail. This can be inferred from a comparison with the peculiar fish leech, Acanthobdella peledina, on the one hand, and with several oligochaetes, like tubificids, enchytraeids and branchiobdellids, on the other hand. The ancestral pattern has been profoundly altered several times in leeches and in oligochaetes. Examples are hirudinids and erpobdellids as well as lumbricids and naidids. The alterations occur in small eggs with little yolk which are nourished by the surrounding fluid within the cocoon. Comparisons with polychaetes lead to the conclusion that an early specification of the 2d cell must be a feature of the annelid ancestor and that equal cleavage represents the derived case in the annelids.