ON THE PREORAL PIT OF THE LARVAL AMPHIOXUS (BRANCHIOSTOMA-LANCEOLATUM)

The ultrastructure of the preoral pit in the highly asymmetric larvae of Branchiostoma lanceolatum was investigated. This organ situated ant erodorsal of the prominent left sided mouth opening is epithelially or ganised with monociliated cells. Its affiliation cowards the classical germ layers is highly variable during ontogeny. Consequently affiliat ion to a germ layer is, in this case, a poor character to base hypothe ses of homology on. Results are discussed against functional and evolu tionary hypotheses from various literature. interpretation of the preo ral pit. as a primitive mouth opening is rejected. No marked specialis ation for glandular activity was discernible but a general secretory a ctivity (as in the entire epidermis) is possible. A sensory function c ould neither-be proved nor rejected. The homology to Rathke's pouch in Vertebrata is probable but its origin from other invertebrates remain s enigmatic. The preoral pit may play a different role in the larval o rganism than its counterpart Hatschek's pit, in the adult.