INTRODUCTION TO PART I. (_Spongillidæ_) 27 The Phylum Porifera 27 General Structure 29 Skeleton and Spicules 33 Colour and Odour 35 External Form and Consistency 37 Variation 39 Nutrition 41 Reproduction 41 Development 45 Habitat 47 Animals and Plants commonly associated with Freshwater Sponges 49 Freshwater Sponges in relation to Man 50 Indian Spongillidæ compared with those of other Countries 51 Fossil Spongillidæ 52 Oriental Spongillidæ not yet found in India 52 History of the Study of Freshwater Sponges 54 Literature 55 GLOSSARY OF TECHNICAL TERMS USED IN PART I. 61 SYSTEMATIC LIST OF THE INDIAN SPONGILLIDÆ 63 INTRODUCTION TO PART II. (_Hydrida_) 129 The Phylum Coelenterata and the Class Hydrozoa 129 Structure of Hydra 130 Capture and Ingestion of Prey: Digestion 133 Colour 134 Behaviour 135 Reproduction 136 Development of the Egg 139 Enemies 139 Coelenterates of Brackish Water 139 Freshwater Coelenterates other than Hydra 141 History of the Study of Hydra 142 Bibliography of Hydra 143 GLOSSARY OF TECHNICAL TERMS USED IN PART II. 145 LIST OF THE INDIAN HYDRIDA 146