30. — — FEMALE REPRODUCTIVE ORGANS • 129 31. — — MALE REPRODUCTIVE ORGANS • 130 32. — — STRUCTURE OF OVARY • 131 33. — — STRUCTURE OF TESTIS • 132 34. — — SPERMATOZOA • 134 35. — — THE LAST THORACIC STERNUM IN THE MALE AND FEMALE • 136 36. — — TRANSVERSE SECTION OF ABDOMEN • 142 37. — — ABDOMINAL APPENDAGES • 144 38. — — CONNECTION BETWEEN THORAX AND ABDOMEN • 151 39. — — CEPHALOTHORACIC STERNA AND ENDOPHRAGMAL SYSTEM • 153 40. — — OPHTHALMIC AND ANTENNULARY SOMITES • 156 41. — — THE ROSTRUM • 157 42. — — A SEGMENT OF THE ENDOPHRAGMAL SYSTEM • 159 43. — — LONGITUDINAL SECTION OF CEPHALOTHORAX • 162 44. — — THE THIRD MAXILLIPEDE • 164 45. — — THE FIRST AND SECOND MAXILLIPEDES • 166 46. — — THE SECOND AMBULATORY LEG • 169 47. — — THE MANDIBLE AND MAXILLÆ • 171 48. — — THE EYE-STALK, ANTENNULE, AND ANTENNA • 172 49. — — BLOOD CORPUSCLES • 176 50. — — EPITHELIUM • 178 51. — — CONNECTIVE TISSUE • 179 52. — — MUSCULAR TISSUE • 181 53. — — MUSCULAR TISSUE • 182 54. — — NERVE GANGLIA • 188 55. — — NERVE FIBRES • 189 56. — — CUTICULAR TISSUE • 191 57. — — SECTIONS OF EMBRYOS • 208 58. — — EARLIER STAGES OF DEVELOPMENT • 210 59. — — LATER STAGES OF DEVELOPMENT • 216 60. — — NEWLY HATCHED YOUNG • 220 61. _Astacus torrentium_, _Astacus nobilis_, and _Astacus nigrescens._ COMPARATIVE VIEWS OF THE CARAPACE, THIRD ABDOMINAL SOMITE, AND TELSON • 233 62. — —, — —, and — — COMPARATIVE VIEWS OF THE FIRST AND SECOND ABDOMINAL APPENDAGES OF THE MALE • 245 63. _Cambarus Clarkii_ • 248 64. _Parastacus brasiliensis_ • 250 65. _Astacoides madagascarensis_ • 251 66. DIAGRAM OF THE MORPHOLOGICAL RELATIONS OF THE _Astacidæ_ • 253 67. _Homarus vulgaris_ • 258 68. _Parastacus_, _Nephrops_, and _Palæmon._ PODOBRANCHIÆ • 259 69. _Nephrops norvegicus_ • 260 70. _Palinurus vulgaris_ • 262 71. _Palæmon jamaicensis_ • 269 72. _Cancer pagurus_ • 273 73. _Penæus_ • 281 74. _Cancer pagurus._ DEVELOPMENT • 282 75. _Astacus leptodactylis_ • 301 76. _Australian Crayfish_ • 307 77. MAP OF THE DISTRIBUTION OF CRAYFISHES • 309 78. _Cambarus._ WALKING LEG • 312 79. _Palæmon jamaicensis_ • 329 80. _Pseudastacus pustulosus_ and _Eryma modestiformis_ • 340 81. _Hoploparia longimana_ • 342 THE CRAYFISH: AN INTRODUCTION TO THE STUDY OF ZOOLOGY. CHAPTER I. THE NATURAL HISTORY OF THE COMMON CRAYFISH (_Astacus fluviatilis._) Many persons seem to believe that what is termed Science is of a widely different nature from ordinary knowledge, and that the methods by which scientific truths are ascertained involve mental operations of a recondite and mysterious nature, comprehensible only by the initiated, and as distinct in their character as in their subject matter, from the processes by which we discriminate between fact and fancy in ordinary life.