Abstract: The radula in the chitons is a ribbon-like apparatus which bears a series of distinctive mineralized teeth called the major lateral teeth.Great amounts of minerals including magnetite have been deposited at the cusps of the teeth.The magnetite mineralization could harden the teeth.In order to investigate the mechanism of the biomineralization of the cusp,the radulae of chiton Acanthochiton rubrolieatus Lischke were extracted and posterior ends of the radulae approximately 0.5 cm in length were fixed,sectioned and observed with light microscope,scanning electron microscope and transmission electron microscope.In addition,aimed to display the iron minerals,the existence and the distribution of the ferric reductase,the sections were examined by histochemistry technology.Results showed that the radula underwent a mineralization progress and the different parts of the radula exhibited various mineralization degrees;the major lateral tooth cusp was a composite structure made up of three distinct layers from the outer to inner.The black magnetite layer was about 50 μm in thickness on the posterior edge,and 5-10 μm on the anterior edge.The lepidocrocite layer in red-brown color below the magnetite layer was only 10 μm in thickness;the core was the yellow organic fibril layer.Magnetite particles which constituted the magnetite layer presented chip-shaped pieces and were 100-150 nm in width,hundreds of nanometers to several micrometers in length;moreover,all the magnetite particles were arranged to be parallel to the length axis of the tooth cusp.Radular teeth including the major lateral teeth were formed by epithelial cells of the radular sac,and the epithelium lining the lumen of the radular sac varied markedly along its length.Newly formed major lateral tooth cusp was almost composed of organic matrix,and the fibrous matrix presented meshwork-like appearance.With the development of mineralization,some fine mineral granules began to be deposit into the organic matrix of mineralizing major lateral tooth.The minerals first appeared in the organic matrix were ferric iron minerals,then ferroc iron minerals.Along the outer surface of mineralizing major lateral tooth cusp surrounded by epithelium of the radular sac,a microvilli-like structure was formed,by which minerals gradually were transported into the cusp.Iron-containing minerals presumably originated from the inclusions which were membrane-bound aggregates of resembling ferritin molecules with 0.6-0.8 μm diameter in surrounding epithelium of the radular sac.The ferric reductase occurred in the epithelial tissue of the radular sac,and mainly distributed on the membrane of these epithelial cells,furthermore,it may be involved in the formation of the magnetite mineralized in the cusp.