closely branched gills on either side of the body where the legs join the shell although the shell must be broken to see them anyone who opens a crab to extract the meat cannot avoid noticing them for they occupy a great part of what seems to be the interior in life however a constant stream of water is passing over them and each gill is in fact an upper branch of one of the legs blood from the heart is circulated through the gills the blood is bluish and has the same copper as in most mollusks