A diagram of a hypothetical ancestral mollusc by Nicholson and Lydekker, altered from an earlier diagram by Lankester. M, mouth; A, anus; L, liver; F, foot; SH, shell, attached to the mantle; H, heart; PE, pericardium; N, kidney (nephridium), opening internally into the pericardium and externally near the anus (K), BR, gill (branchia); CG, cerebral ganglion; PG, pleural ganglion; FG, pedal ganglion; VG, visceral ganglion; VN, visceral nerve cord.