Animals obtain nutrients using a two-step procedure: food collection and uptake by endocytosis. In eumetazoans, a particular intestinal tract evolved, making it possible to digest food prior to the uptake of the smallest compartments. This chapter presents models for the evolution of this intestinal tract and its further evolution. There is a general trend for evolution from a saclike system to a tubelike one-way gut. In particular, the anterior, ectodermal part of the intestinal tract can be very diverse and contains a pharynx in various forms or it contains cuticular hard structures. Feeding is discussed in the context of larvae evolution and in adults.
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