It is through this passage, or indirectly through the pores of the gill-plates, that the water introduced into the lower subpallial space must pass on its way to the excurrent siphonal notch.
The atrial region extends from the mouth over about twothirds of the length of the body, terminating at a large median ventral aperture, the atriopore; this is the excurrent orifice for the respiratory current of water and also serves for the evacuation of the generative products.