1. The course of the water currents in the mantle cavity of three species of the Chitonida, and one species of the Lepidopleurida, has been determined.
2. Inhalant openings are created anteriorly or laterally by local raising of the girdle. The single exhalant opening is always posterior and confined to the region between the last pair of gills.
3. The exhalant current carries with it the genital and excretory products, and, in the Chitonida, the faeces.
4. The bridging of the pallial grooves in the region of the girdle folds by the post-renal gills (and adanal gills in the Lepidopleurida) completes the functional division of the pallial grooves into inhalant and exhalant chambers.
5. The gills possess the typical structure of ctenidia, and their ciliation is a modification only of that of ctenidia. They are to be regarded as multiplied ctenidia and not as secondary structures.
6. The individual filaments are shortened, attached to those of adjacent gills by long interlocking cilia, and have a broad band of lateral cilia which create the respiratory current.
7. Four possible tracts of mucous glands in the pallial grooves are concerned with the consolidation of sediment. The pallial tracts may be homologous with the hypobranchial glands of the Prosobranchia; all are analogous with these. In the Chitonida sediment is rejected only from the exhalant chamber, in the Lepidopleurida mainly from the inhalant chamber.
8. Osphradia, possibly homologous with those in the Gastropoda, occur in the majority of the Chitonida. With them may be associated anterior sense organs. In the Lepidopleurida they are replaced by branchial and lateral sense organs. All are similar in structure and innervation. They have been considered olfactory in function, but with equal reason may be regarded as tactile organs concerned with the estimation of sediment.
9. The Loricata probably evolved between tide-marks, their characteristic structure being admirably adapted for life on the shore.
10. The reasons for the differences between the structure and habits of the Lepidopleurida and the Chitonida are discussed.