Olfactometer
There are two chemical sensory organs for those with two comb gills, and only one chemical sensory organ for those with one comb gill. It is a kind of fold or depression of sensory epithelium, and there are many leaflike parts on the left and right sides of the central prismatic part of the gill of some axopods. Because of the similarity with the structure of the comb gill, it is called the parabranc hia
