The outer margins of the lamellae from adjacent filaments come into close proximity to each other completing
a sieve-like arrangement for the respiratory water
to pass through. There is a space left between the
trailing edge of the lamellae and the lateral wall of
the interbranchial septum so the water that has
passed between the lamellae can flow on out
behind the rows of lamellae. This water channel
or canal facilitates the flow of water counter to the
lamellar blood flow.