Systematics or Phylogenetics

This species belongs to the class Turbellaria, part of the phylum Platyhelminthes which also includes parasitic flukes (Trematoda) and tapeworms (Cestoda). The class Turbellaria consists of numerous orders of free-living flatworms, however there is some debate as to whether this is a natural grouping and the class appears to be paraphyletic⁷. Polycladida has traditionally been regarded as an order of the Turbellaria, and the unidentified leptoplanid flatworm belongs to this order. The polyclads are often relatively large and conspicuous, and may be distinguished from other turbellarians by their complex branching gut system. The polyclads may be further divided into two suborders, the cotyleans which have a ventral sucker, and the acotyleans which do not. The Leptoplanidae belong to the latter suborder.

Platyhelminthes are often considered to be some of the earliest bilaterians, however there has been much debate over the evolutionary history of the Turbellarians, and there are currently two major theories: the ‘planula theory’ and the ‘coelomate theory’⁷. The planula theory suggests that turbellarians and cnidarians evolved from a common ancestor resembling the planula larvae of the cnidaria. The coelomate theory suggests that turbellarians evolved from a more complex, coelomate ancestor into their current ‘primitive’ form through simplification. However, so far the systematic placement of the Turbellaria has yet to be fully resolved⁷.