Turbellarians display a number of behaviors that prevent them from straying beyond their normal habitats and allows to them to maintain orientation within those habitats. For instance, most turbellarians are positively thigmotactic (touch) ventrally and negatively thigmotactic dorsally. This allows them to maintain their ventral side against the substrate in benthic forms. In other species where touch may not be the best way to orient to a substrate (such as interstitial and pelagic
forms), they have statocysts so that they can orient to gravity (geotaxis). Most species are also negatively phototactic, which prevents worms from coming out in the daylight where they may get eaten or dry out in the case of terrestrial forms.
All turbellarians have a strong sense of smell that can be used to find food or mates (chemotaxis). Chemosensors are concentrated on each side of the head to help them determine the direction that the chemical trail is coming from. The heads of freshwater species are often expanded into auricles that have sensors. Some species have tentacles and ciliated pits to assist in chemotaxis. Dugesia swings its head back and forth to help determine the proper direction of the food source. Other species use trial and error to determine the proper direction to find food. They move in one direction until the signal gets weaker, and then continue switching direction until the signal is strongest. Some species also have been shown to orient to currents in order to find food (rheotaxis).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.