The micrograph below of a fish's gill clearly shows the secondary lamellae arranged at 90degrees to the gill filaments  

a i Using information from the micrograph explain why the structure of the gills make them efficient at exchanging gases between blood and water  
  ii The flow of blood and water through the secondary lamellae is often described as countercurrent. Explain the significance of this in relation to gas exchange  
b   Name the structure labelled X in the diagram below