Chapter 6 Flatworms Turbellarians and Nemertea

The turbellarian flatworms are basal among the Bilateria, showing such presumably primitive features as an incomplete gut—with only a mouth and no anus—and an acoelomate body structure. Most turbellarians are members of the phylum Platyhelminthes, which also comprises the major parasitic groups of flatworms—namely the Monogenea (ectoparasitic flukes), Cestoidea (tapeworms), and Trematoda (endoparasitic flukes)—and which is considered part of the major Bilaterian group Lophotrochozoa. More than 200 species of turbellarians and 2–3 species of nemerteans live in freshwaters of North America. Even though they are important predators in freshwater ecosystems and are often common in relatively high numbers, their ecology and systematics have been less studied than that of many other common aquatic invertebrates. This limited attention is partly due to difficulty in identifying and preserving such soft-bodied animals. Most turbellarians become unrecognizable in routinely preserved field samples. Therefore, the study of live specimens, often in conjunction with histological study of individually processed specimens, is the only reliable means for identifying species. Accurate identification of nemerteans also requires histological study. Considering their relative obscurity, many interesting discoveries lie ahead concerning the reproductive biology, dispersal, endosymbiosis, and community structure of these worms. This chapter describes the general biology and classification of the more than 200 species of turbellarians (phylum Platyhelminthes) and two-to-three species of ribbon worms (phylum Nemertea, or Rhynchocoela) found in North American lakes and streams in shallow to deep waters.