I found the movement of the carmine dye along the ctenidia of the Mytilus galloprovincialis pretty fascinating. This first video shows how the dye moves along the demibranch and then along the groove of the ctenidia towards the mouth. The 2nd video shows one of the two globs of dye (and some other material – mucus?)that accumulated near the mouth. (See the picture below of the mussel after I first opened it and how the globs weren’t there).
Although it’s hard to see, this picture shows the very fine grooves or lines in the ctenidium.
This last picture is of a small piece of the ctenidum under the compound microscope.