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Symbiosis is a dominant trait in the Platyhelminthes. The Neodermata (Aspidogastrea, Monogenea, Digenea, Udonellidea, Cestoda) are wholly parasitic and even the predominantly free-living Turbellaria have almost 200 species from 35 families living in permanent associations with other animals. In the simplest turbellarian symbioses, ectosymbiotes such as the Temnocephalida, some other Rhabdocoela and a few Tricladida live on the body surfaces or in the branchial chambers of their mainly arthropodan or chelonian hosts. They feed on the same types of prey as their free-living relatives but supplement their diet by opportunistic commensalism. Their digestive physiology and food reserves are the same as in free-living species. The entosymbiotic Umagillidae, Graffillidae, Pterastericolidae, Fecamplidae and Acholadidae live in internal body cavities or body wall derivatives of echinoderms, molluscs or arthropods and show increasing metabolic dependence on their hosts. Patterns of digestive physiology and food storage generally differ markedly from those of ectosymbiotic and free-living species. Some umagillids, in echinoids, feed as entozoic predators on co-symbiotic protozoa, supplemented by opportunistic ingestion of the hosts' ingesta, gut cells or coelomocytes. Others, in holothurians, feed mainly on gut cells, which also provide some digestive enzymes, and to a lesser extent on host ingesta and co-symbiotes. Graffillids, in molluscs, lack endogenous digestive enzymes and rely entirely on those taken in with host ingesta and gut tissues. Pterastericolids, in asteroids, similarly utilise gut tissues both as food and enzyme sources. The climax to metabolic dependence occurs in the Fecamplidae and Acholadidae. The former, in crustacean haemocoels and myzostomid tissues, lack conventional alimentary systems and absorb soluble nutrients through the epidermis. In the latter the only known species lives in the tube feet of its asteroid host, lacks a normal endodermal gut, but has a modified epidermis performing both digestive and absorptive functions. Most of these entosymbiotes show a shift from the lipid storage characteristic of free-living and ectosymbiotic species to the glycogen storage predominating in the Neodermata. In both groups this emphasis on carbohydrate metabolism is often independent of the PO2 of their environment. Both groups also show high fecundity and it is suggested that there is a direct relationship between this and glycogen storage. High fecundity, while clearly of adaptive value in entosymbiotes, is arguably primarily related to the assured food supply conferred by the entosymbiotic habit and thus can be viewed as a consequence of the latter rather than a prerequisite for it. Some entosymbiotic Turbellaria have evolved physiologically active haemoglobins, allowing them to abstract oxygen preferentially from host tissues; some have also evolved facultative glycolytic mechanisms comparable to those of the Cestoda. All these adaptations to ecto- and entosymbiotic life in the Turbellaria exemplify possible pathways to wholly parasitic lifestyles, with total metabolic dependence on the hosts, which may have been followed during the evolution of the Neodermata.

1. DDT metabolism in a North American species of planarian leads to the formation of metabolites more toxic than the parent compound. 2. The increased toxicity of DDT metabolites is similar to acute toxicity data reported previously in a European species. 3. It is suggested that planarians lack a direct mechanism for DDT detoxification, since two North American and one European species are known to metabolize DDT initially to DDE and DDD.