3. Pedalia 8 4. Sensory Clubs 9 5. The Bell Cavity and its Structures 10 (a) Proboscis 11 (b) Suspensoria, or Mesogonia 11 (c) Interradial funnels, or funnel cavities 11 (d) Velarium 12 (e) Frenula 12 (f) Musculature 12 (g) Nerve ring 13 c. Internal Anatomy 13 6. Stomach 13 7. Phacelli 14 8. Peripheral Part of the Gastro-Vascular System 14 (a) Stomach Pockets (Valves and Mesogonial Pockets) 14 (b) Marginal Pockets 17 (c) Canals of the Sensory Clubs and Tentacles 17 9. Reproductive Organs 19 10. Floating and Wandering Cells 20 B. TRIPEDALIA CYSTOPHORA 22 a. Habitat 22 b. External Anatomy 23 c. Internal Anatomy 24 PART III: DESCRIPTION OF SPECIAL PARTS OF THE ANATOMY 27 A. VASCULAR LAMELLÆ 27 B. NERVOUS SYSTEM 37 LITERATURE 57 TABLE OF REFERENCE LETTERS 58 DESCRIPTION OF FIGURES 60 INTRODUCTION. Jelly-fish offer to the lover of natural history an inexhaustible store of beauty and attractiveness. One who has studied them finds within him a ready echo to Haeckel’s statement that when first he visited the seacoast and was introduced to the enchanted world of marine life, none of the forms that he then saw alive for the first time exercised so powerful an attraction upon him as the Medusæ. The writer counts it a rare stroke of fortune that he was led to the study of a portion of the group by the discovery of two new species of Cubomedusæ in Kingston Harbor, Jamaica, W. I., while he was with the Johns Hopkins Marine Laboratory in June of 1896.