20.1 Introduction
20.2 Phylogeny and Classification
20.2.1 Sister Group Relationships
20.2.2 Chromosome Numbers
20.3 Morphology
20.3.1 Shell and Operculum
20.3.1.1 Shell Structure
20.3.1.2 Larval or Embryonic Shell
20.3.1.3 Shell Reduction and Loss
20.3.1.4 Shell Variation
20.3.1.5 Operculum
20.3.2 External Body Morphology
20.3.2.1 Head Tentacles
20.3.2.2 Cephalic (Head) Shield
20.3.2.3 Epithelial Modifications
20.3.2.4 Oral Veil
20.3.2.5 The Foot, Pedal Glands, and Parapodia
20.3.2.6 The Mantle
20.3.3 Mantle Cavity and Associated Structures
20.3.3.1 Heterobranch Gills
20.3.3.2 Opposed Ciliary Ridges (Raphes) and Mantle Tentacles
20.3.3.3 The Hypobranchial Gland
20.3.3.4 The Pigmented Mantle Organ (PMO)
20.3.3.5 Other Glands Associated with the Mantle Cavity
20.3.4 Digestive System
20.3.5 The Renopericardial System
20.3.6 Nervous System and Sense Organs
20.3.6.1 Sense Organs
20.3.7 Reproductive System
20.3.7.1 Terminology
20.3.7.2 The Ovotestis and Gametes
20.3.7.3 The Reproductive System of ‘Lower Heterobranchs’
20.3.7.4 Some Small Euthyneurans Previously Thought to Be Lower Heterobranchs
20.3.7.5 Euthyneuran Reproductive Systems
20.4 Biology, Ecology, and Behaviour
20.4.1 Habits and Habitats
20.4.2 Locomotion
20.4.3 Food and Feeding
20.4.3.1 Herbivory
20.4.3.2 Fungivory
20.4.3.3 Active Carnivory
20.4.3.4 Parasitism
20.4.3.5 Suspension Feeding
20.4.3.6 Coprophagy
20.4.4 Reproduction, Development, and Growth
20.4.4.1 Self-Fertilisation
20.4.4.2 Hypodermic Insemination
20.4.4.3 Aphally
20.4.4.4 Asexual Reproduction
20.4.4.5 Life Cycles
20.4.4.6 Reproductive Behaviour
20.4.4.7 Development
20.4.4.8 Spawn
20.4.4.9 Larvae and Larval Shells
20.4.5 Behaviour
20.4.6 Predation and Defence
20.4.6.1 The Shell as a Means of Defence
20.4.6.2 Mucus
20.4.6.3 Defence without a Shell
20.4.6.4 Use of Nematocysts
20.4.6.5 Aposematic (Warning) Colouration and Batesian and Müllerian Mimicry
20.4.6.6 Autotomy
20.4.6.7 Predators of Heterobranchs
20.4.7 Associations
20.4.8 Economic and Ecological Values
20.5 Diversity and Fossil History
20.5.1 Major Groups within the Heterobranchia
20.5.1.1 ‘Lower Heterobranchs’
20.5.1.2 Euthyneura