The ancestral cleavage pattern of the clitellates and its phylogenetic deviations

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