Probe passed into the superior division of the subpallial chamber through the excurrent siphonal notch, and issuing by the side of the foot into the inferior division of the sub-pallial chamber.
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.
Megalodontidae.-Shell 1a, tr, Upper and lower inequilateral, thick; posterior siphons adductor impression on a myo ms, Siphonal muscle of the phorous apophysis.
Mytilus possesses no siphonal tube-like productions of the margin of the mantle-skirt, nor any notching of the same, representative of the siphons which are found in its fresh-water ally, the Dreissensia polymorpha.
A probe (g) is introduced through this slit-like passage, and it is seen to pass out by the excurrent siphonal notch.