| Meanwhile, Pelseneer had come up with a classification of bivalves based on the structure of the ctenidia. |
| No ctenidia but pallial branchiae in a circle between mantle and foot. |
| A single pair of ctenidia is present instead of the two pairs in Nautilus. |
| No ctenidia are present, and gas exchange is through the mantle surface. |
| Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. |
| All cephalopods have one pair of unciliated ctenidia within the mantle cavity, with the exception of Nautilus, which has two pairs of ctenidia. |
