1. Transverse section of an Actinia (Agassiz) 5 2, 3, 4. Actinia in different degrees of expansion (Agassiz) 8 5. METRIDIUM MARGINATUM fully expanded 8 6. Vertical section of an Actinia 10 7. View from above of an expanded Actinia 11 8, 9. Young Actiniæ 11 10. RHODACTINIA DAVISII 13 11. ARACHNACTIS BRACHIOLATA 14 12. Young Arachnactis 14 13. Young Arachnactis showing the mouth 14 14. BICIDIUM PARASITICUM 15 15. HALCAMPA ALBIDA 16 16. Colony of ASTRANGIA DANÆ 17 17. Magnified individuals of Astrangia 17 18. Single individual of Astrangia 18 19. Lasso-cell of Astrangia 18 20. Limestone pit of Astrangia 19 21. Single individual of HALCYONIUM CARNEUM 19 22. Halcyonium community 20 23. Expanded individual of Halcyonium 20 24. Branch of MILLEPORA ALCICORNIS (Agassiz) 22 25. Expanded animals of Millepora (Agassiz) 22 26. Transverse section of branch of Millepora (Agassiz) 23 27. PLEUROBRACHIA RHODODACTYLA (Agassiz) 27 28. The same as Fig. 27 seen in plane of tentacles (Agassiz) 28 29. Pleurobrachia in motion 29 30. Pleurobrachia seen from the extremity opposite the mouth 30 31. BOLINA ALATA seen from the broad side (Agassiz) 31 32. Bolina seen from the narrow side (Agassiz) 31 33. IDYIA ROSEOLA seen from the broad side (Agassiz) 32 34. Young Pleurobrachia still in the egg 35 35. Young Pleurobrachia swimming in the egg 35 36. Young Pleurobrachia resembling already adult 35 37. Young Idyia 35 38. Young Idyia seen from the anal pole 36 39. Idyia somewhat older than Fig. 37 36 40. Idyia still older 36 41. Young Bolina in stage resembling Pleurobrachia 37 42. Young Bolina seen from the broad side 37 43. Young Bolina seen from the narrow side 37 44. CYANEA ARCTICA 40