FIG. PAGE 



27. Metridium Cut Through Vertically Except for Its Oral Disc 95 



28. Metridium Cut Through Vertically Except for Its Aboral Disc 95 



29. Melridium Cut Through Vertically Except for a Small Connecting 



Bridge on the Pedal Edge of the Column 96 



30. Section at Right Angles to the Sphincter of the Bell of Rhizostoma. . 105 



31. Diagram of the Jellyfish Aurelia from Which the Central Mass and 



Seven of the Eight Marginal Bodies Have Been Removed and 

 the Outline of the Bell Further Complicated by a System of 

 Interdigitated Cuts 107 



32. Diagram of the Jellyfish Aurelia from Which the Central Mass and 



Seven of the Eight Marginal Bodies Have Been Removed and 

 the Outline of the Bell Further Complicated by a Circular 

 Incision 108 



33. Diagram of a Portion of the Bell of Rhizostoma 110 



34. Preparation of Two Muscle Fields from Rhizostoma Ill 



35. Preparation of Two Muscle Fields from Rhizostoma, One With a Mar- 



ginal Bod}' and the Other Without Such a Body 113 



36. Nerve Network from a Small Blood-vessel in the Palate of the Frog. 118 



37. Diagram of the Nervous Organization in the Intestinal Wall of a 



Vertebrate 128 



38. Diagram of the Reactions of a Tentacle of a Sea-anemone to Tran- 



section 131 



39. Sense Cells With Their Attached Fibers in a Tentacle of the Sea- 



anemone Cerianthus 132 



40. Diagram to Illustrate the Preparation of the Intestine in Mall's 



Experiment on Intestinal Polarization 134 



41. Outline and Transverse Section of the Sea-anemone Meiridium 136 



42. Diagram of a Jellyfish Aurelia Whose Bell Has Been Deeply Incised 



in a Radial Direction at Eight Places 166 



43. Side View of the Hydroid Corymorpha 176 



44. A Polyp of Corymorpha Split Lengthwise Except at Its Aboral End . 188 



45. Diagram of an Independent Effector 200 



46. Diagram of a Simple Type of Receptor-effector System 201 



47. Diagram of a Complex Type of Receptor-effector System 202 



48. Transverse Section of the Ventral Nerve-chain of the Marine Worm 



Sigalion 203 



49. Transverse Section of the Ventral Nerve-chain of the Earthworm 



Allolobophora 205 



50. Diagram of a Transverse Section of the Spinal Cord of a Vertebrate 



(Salamander) 206 



51. Stages in the Differentiation of Sense Cells 210 



52. Stages in the Differentiation of Nerve Cells 211 



53. The Appropriation of Secondary Sensory Cells by Primary Sensory 



Neurones in the Vertebrates 213 



12 



