The Aspidogastrea: An archaic group of Platyhelminthes

The Aspidogastrea is a small group of platyhelminths comprising four families, 12 genera and approximately 80 species. Among the genera, 'Sychnocotyle' from Australian freshwater turtles was described recently (Ferguson et al 1999), a second one, 'Rohdella', from freshwater teleosts in Southeast Asia, 15 years ago (Gibson and Chinabut 1984), and another, 'Rugogaster', from the rectal glands of chimaerid fishes in the northern Pacific, 26 years ago (Schell 1973), indicating that the taxonomy of the group is unlikely to be completely known (for a recent taxonomic discussion see Rohde in press). The main characteristics of the larvae are illustrated in Figure 15.1. The larvae of all species that have been examined have a posterior sucker, a pharynx and a simple or bifurcate intestine. Some species have a false oral sucker (not lined by a capsule but separated from the parenchyma of the main body by a septum extending from the pharynx to the tegument), others lack such a sucker. Excretory pores are paired and located at the level of the anterior margin of the posterior sucker, connected to two excretory ducts each with three flame bulbs in those species that have been examined for this character (and lateral flames, described only in larval 'Multicotyle purvisi'). Some species have a short posterior appendage, some have eye-spots, and some have ciliary patches; other species lack these characters (for a detailed discussion and references see Rohde 1972, 1994a).