We have examined with light and electron microscopy various species belonging to the three families of appendicularians (Oikopleuridae, Fritillariidae, Kowalevskiidae). All species show a simplification of trunk organs, that is extreme in Fritillariidae and Kowalevskiidae. We renconstructed the digestive apparatus, using specimens cut with complete series of sections. No connective or muscle tissue are associated with the gut epithelium and no cell renewing regions, nor endocrine-like cells were recognised. The gut is composed of a simple epithelium and bathed by an internal fluid lacking blood cells. The gut tracts are separated by valves (usually cardiac, pyloric and rectal); however, the relative tract extension varies among species. In the genus Oikopleura several cells with different functions are recognisable, whereas in the other families cells are very few, mostly giant and poorly diversified. Food progresson depends upon ciliary activity and the gut wall has morphological and enzymatic signs of process of absorption, digestion and epithelial transport. The endostyle varies form Oikopleura to Fritillaria and is absent in Kowalevskia. Oikopleurid species possess common characters in the histology of the alimentary tract, completely distinct form fritillarids and kowalevskids, which, on the other hand, have other characters in common.