Mucus secretion and transport in amphioxus larvae: organization and ultrastructure of the food trapping system, and implications for head evolution

Abstract

AbstractA low‐to‐medium power transmission electron microscopy survey of the larval head of amphioxus was used to re‐examine the position and constituent cell types of the mucus‐secreting organs involved in feeding. Previously unreported features include cells with fibril‐filled paraciliary processes in the recessed (pit) portion of the preoral organ, which probably assist in generating the mucus string produced by this structure, and the cell responsible for driving the current through the club‐shaped gland, which appears to depend on a mechanism analogous to Archimedes’ screw. Pharyngeal structures are dramatically repositioned during larval growth and metamorphosis. Mapping these changes shows that they are most easily explained if the positioning of the mouth is not directly controlled by the mechanism used to pattern the rest of the ventral pharynx. This accords with the predictions of the dorsoventral inversion hypothesis, which requires that an originally dorsal mouth in the inverted chordate ancestor be secondarily shifted to the ventral surface. It is argued here, on this basis, that the repositioning of the larval mouth in amphioxus, from the left side to the ventral midline, represents a partial recapitulation of past evolutionary events.