4.1 Introduction
4.2 The Mantle Cavity
4.2.1 Mantle Cavity Structures
4.2.1.1 Ctenidia
4.2.1.2 Reduction and Loss of Ctenidia
4.2.1.3 Secondary Gills and Plicatidia
4.2.1.4 Opposed Ciliated Ridges
4.2.1.5 Modification as a Lung
4.2.1.6 Hypobranchial Glands
4.2.1.7 Pigmented Mantle Organ (PMO)
4.2.1.8 Sensory Structures
4.2.1.9 Openings to the Mantle Cavity
4.2.2 Respiration, Ventilation, and Waste Management
4.2.2.1 Respiration
4.2.2.2 Ventilation
4.2.2.3 Waste Removal
4.2.3 Symbionts