The Crayfish: an Introduction to the Study of Zoology.

CHAPTER VI.. The Distribution and the ætiology of the Crayfishes • 288

The Distribution and the Ætiology of the Crayfishes • 288

NOTES • 347

BIBLIOGRAPHY • 357

INDEX • 363

LIST OF WOODCUTS.

Frontispiece. THE COMMON CRAYFISH, Astacus fluviatilis, (MALE)

1. Astacus fluviatilis. SIDE VIEW OF THE MALE • 6

2. — — DORSAL VIEWS OF MALE AND FEMALE • 18

3. — — VENTRAL VIEWS OF MALE AND FEMALE • 21

4. — — THE GILLS • 26

5. — — DISSECTION FROM THE DORSAL SIDE (MALE) • 28

6. — — LONGITUDINAL VERTICAL SECTION OF THE ALIMENTARY CANAL • 29

7. — — A GASTROLITH OR “CRAB’S EYE” • 30

8. — — ATTACHMENT OF YOUNG TO SWIMMERET OF MOTHER • 41

9. — — STRUCTURE OF THE STOMACH • 53

10. — — LONGITUDINAL SECTION OF THE STOMACH • 56

11. — — ROOF OF THE STOMACH, FROM WITHIN • 60

12. — — DISSECTION FROM THE SIDE (MALE) • 62

13. — — ALIMENTARY CANAL FROM ABOVE • 65

14. — — BLOOD CORPUSCLES • 68

15. — — TRANSVERSE SECTION OF THORAX • 70

16. — — THE HEART • 72

17. — — STRUCTURE OF THE GILLS • 76

18. — — THE GREEN GLAND • 83

19. — — MUSCULAR TISSUE • 91

20. — — MUSCLES OF CHELA • 93

21. — — ARTICULATION OF ABDOMINAL SOMITES • 97

22. — — MUSCULAR SYSTEM • 100

23. — — NERVE FIBRES • 102

24. — — NERVE GANGLIA • 103

25. — — NERVOUS SYSTEM • 104

26. — — OLFACTORY AND AUDITORY ORGANS • 114

27. — — AUDITORY SAC • 117

28. — — STRUCTURE OF EYE • 119

29. — — DIAGRAM OF EYE • 123

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

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.

But any one who looks into the matter attentively will soon perceive that there is no solid foundation for the belief that the realm of science is thus shut off from that of common sense; or that the mode of investigation which yields such wonderful results to the scientific investigator, is different in kind from that which is employed {2} for the commonest purposes of everyday existence. Common sense is science exactly in so far as it fulfils the ideal of common sense; that is, sees facts as they are, or, at any rate, without the distortion of prejudice, and reasons from them in accordance with the dictates of sound judgment. And science is simply common sense at its best; that is, rigidly accurate in observation, and merciless to fallacy in logic.

Whoso will question the validity of the conclusions of sound science, must be prepared to carry his scepticism a long way; for it may be safely affirmed, that there is hardly any of those decisions of common sense on which men stake their all in practical life, which can justify itself so thoroughly on common sense principles, as the broad truths of science can be justified.

The conclusion drawn from due consideration of the nature of the case is verified by historical inquiry; and the historian of every science traces back its roots to the primary stock of common information possessed by all mankind.

In its earliest development knowledge is self-sown. Impressions force themselves upon men’s senses whether they will or not, and often against their will. The amount of interest which these impressions awaken is determined by the coarser pains and pleasures which they carry in their train, or by mere curiosity; and reason deals with the materials supplied to it as far as that interest carries it, and no farther. Such common {3} knowledge is rather brought than sought; and such ratiocination is little more than the working of a blind intellectual instinct.

It is only when the mind passes beyond this condition that it begins to evolve science. When simple curiosity passes into the love of knowledge as such, and the gratification of the æsthetic sense of the beauty of completeness and accuracy seems more desirable than the easy indolence of ignorance; when the finding out of the causes of things becomes a source of joy, and he is counted happy who is successful in the search; common knowledge of nature passes into what our forefathers called Natural History, from whence there is but a step to that which used to be termed Natural Philosophy, and now passes by the name of Physical Science.

In this final stage of knowledge, the phenomena of nature are regarded as one continuous series of causes and effects; and the ultimate object of science is to trace out that series, from the term which is nearest to us, to that which is at the furthest limit accessible to our means of investigation.

The course of nature as it is, as it has been, and as it will be, is the object of scientific inquiry; whatever lies beyond, above, or below this, is outside science. But the philosopher need not despair at the limitation of his field of labour: in relation to the human mind Nature is boundless; and, though nowhere inaccessible, she is everywhere unfathomable. {4}

The Biological Sciences embody the great multitude of truths which have been ascertained respecting living beings; and as there are two chief kinds of living things, animals and plants, so Biology is, for convenience sake, divided into two main branches, Zoology and Botany.

Each of these branches of Biology has passed through the three stages of development, which are common to all the sciences; and, at the present time, each is in these different stages in different minds. Every country boy possesses more or less information respecting the plants and animals which come under his notice, in the stage of common knowledge; a good many persons have acquired more or less of that accurate, but necessarily incomplete and unmethodised knowledge, which is understood by Natural History; while a few have reached the purely scientific stage, and, as Zoologists and Botanists, strive towards the perfection of Biology as a branch of Physical Science.

Historically, common knowledge is represented by the allusions to animals and plants in ancient literature; while Natural History, more or less grading into Biology, meets us in the works of Aristotle, and his continuators in the Middle Ages, Rondoletius, Aldrovandus, and their contemporaries and successors. But the conscious attempt to construct a complete science of Biology hardly dates further back than Treviranus and Lamarck, at the beginning of this century, while it has received its strongest impulse, in our own day, from Darwin. {5}

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