It is known that amphioxus in the course of its life history passes through a free swimming larval phase and a sedentary adult phase when it lies half buried in sand. The notochord is known to be soft and flexible in the larval stages and is hard and rigid in the adult condition. Similarly the gill bars show a change from a soft flexible condition to a hard and rigid structure as it passes from the larval to adult phase. Such changes are correlated with the requirements for their survival in the two respective| phases. The transition from one condition to the other has been shown to be due to changes in the chemical composition of the respective structures involving a process of tanking in which the protein constituents of the gill bar and the notochord undergo first a condensation with a lipid to form a lipoprotein complex and later form cross linkages with quinones formed by the oxidation of phenolic substances present with the protein. The process strongly recalls the tanning of the arthropod cuticle resulting in mechanical rigidity and chemical resistance which are of functional significance to the animal in the larval and adult phases.