The phylum Platyhelminthes (Greek, platyflat, helminthesworm) includes at least 30 000 species. The phylum represents a large and diverse group of organisms, most of which are obligate parasites, living on, or in, most species of vertebrate and invertebrate animal. As the phylum name suggests, these worms are flattened dorsoventrally. They are without segmentation, although cestodes, or tapeworms, superficially appear otherwise. Cestodes are modular iterations, with each segment or proglottid being more like an individual within a colony since each is a complete sexual unit (Hughes, 1989). Moreover, there is no coelom or peritoneum as there are in truly coelomate, segmented animals such as the annelids. Platyhelminths may, or may not, possess an incomplete gut. They are without circulatory, skeletal, and respiratory systems. The functional and structural unit of their excretory/osmoregulatory system is a protonephridium, or flame cell (Fig. 6.1), so named for a tuft of cilia extending away from the cell body that resembles the flame of a burning candle. Most species are monoecious, but a few, such as the medically important schistosomes, are dioecious.
Parasitic platyhelminths are extraordinarily diverse in terms of their morphology, habitats, life cycles, and transmission adaptations. The ectoparasitic monogeneans of primarily fish and amphibians, for example, have direct life cycles, featuring a free-swimming oncomiracidium stage. On the other hand, all endoparasitic digenean trematodes (flukes) have remarkably complex life cycles with molluscs (mostly snails) as first intermediate hosts, in which free-swimming stages known as cercariae are produced by extensive asexual reproduction. Further, most trematodes (and many cestodes) incorporate a resting stage within a second intermediate host. This host is generally a potential prey item of the definitive vertebrate host in which the parasite matures. Thus, most trematodes and cestodes are transmitted via predator–prey interactions. To add to the complexity, and stressing the importance of trophic transmission and food web dynamics, still other trematodes and cestodes have added third intermediate, or often, paratenic hosts to their life cycles. How and why such life cycle complexity evolved in the Platyhelminthes has long been the subject of debate (reviews in Cribb et al., 2003; Parker et al., 2003). As we will see in this, and several subsequent chapters, many species of platyhelminth are utilized as model systems for addressing questions in ecological, evolutionary, and environmental parasitology (see Chapters 12–17).