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  
    Structure Explanation  
    large surface area fast diffusion  
    thin/small gap between blood and water fast diffusion  
    many capillaries in lamellae maintain diffusion gradients  
    NOTE max of 2 for structures and max of 2 for explanations (4)
  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  
  • diffusion takes place over full length of gill lamellae
  • therefore final percentage saturation is greater
b   Name the structure labelled X in the diagram below  

    operculum/opercular flap (1)