The appendicularian alimentary tract: a comparative study

Species belonging to the three families of appendicularians (Oikopleuridae, Fritillariidae, Kowalevskiidae) reveal general simplification of trunk organs, especially Fritilalriidae and Kowalevskiidae. In particular, the alimentary tract, studied by light and electron microscopy using specimens cut with complete series of sections, shows that no connective or muscle tissue accompanies the gut epithelium, and no cell-renewing regions nor endocrine-like cells can be identified. In the pharynx, the endostyle varies in complexity from Oikopleura to Fritillaria, but is absent in Kowalevskia. The post-pharyngeal tract is always composed of an oesophagus, a stomach and an intestine, the latter subdivided into two or three regions. In Oikopleura, several cell types with different functions are recognisable. In the other families, the cells are very few, mostly giant, and poorly diversified. Food passage depends upon ciliary activity. Absorption, digestion and epithelial transport are also evident. The gut tracts are always separated by valves (usually cardiac, pyloric and rectal) although their importance varies among species. In general, oikopleurids possess common features in the histology of the alimentary tract, completely distinc from fritillarids and kowalevskids, which have othe features in common.