Protonephridia as phylogenetic characters

It is essential for a cladistic analysis that the only characters to be used are those likely to be homologous (e.g., Rohde 1990, 1996, further references therein). Rieger and Tyler (1985) summarized homology criteria as follows: 1) homologous structures have a similar position relative to other structures, as well as similarities in the position of substructures; 2) even dissimilar structures may be homologous if they are connected by a sequence of intermediate forms; 3) structures can be interpreted as homologous if their distribution in a group of organisms coincides with other similarities; the first and most important criterion should always be used first. A homology analysis should be supplemented by a functional analysis using the following criteria (Rieger and Tyler 1985): 4) convergence of similar structures becomes more likely with increasing correlation between the similarity of structures and certain environmental conditions; 5) convergence becomes less likely if there are many 'solutions' to a certain problem; 6) different ontogenetic origins of structures suggest convergence; and 7) common selection pressure suggests convergence. In the following, I examine the usefulness of protonephridial ultrastructure for establishing phylogenetic relationships in the platyhelminths using the above homology criteria. Previous discussions are by Ehlers (1985a), Ehlers and Sopott-Ehlers (1986), Rohde (1990, 1991, a brief account also in 1988), and (Rohde et al. 1995), but the vastly improved database now available makes a new evaluation necessary. Ruppert and Smith (1988) discussed the functional organization of filtration nephridia in various animal groups, and recent discussions of function and evolution of protonephridia including those of platyhelminths are by Bartolomaeus and Ax (1992) and Xylander and Bartolomaeus (1995). A brief, somewhat out-of-date account was provided by Hertel (1993).