In the Appendix, I have added a few notes respecting points of detail with which I thought it unnecessary to burden the text; and, under the head of Bibliography, I have given some references to the literature of the subject which may be useful to those who wish to follow it out more fully. I am indebted to Mr. T. J. Parker, demonstrator of my biological class, for several anatomical drawings; and for valuable aid in supervising the execution of the woodcuts, and in seeing the work through the press. Mr. Cooper has had charge of the illustrations, and I am indebted to him and to Mr. Coombs, the accurate and skilful draughtsman to whom the more difficult subjects were entrusted, for such excellent specimens of xylographic art as the figures of the Crab, Lobster, Rock Lobster, and Norway Lobster. T. H. H. LONDON, _November, 1879_. CONTENTS. PREFACE • v LIST OF WOODCUTS • xi CHAPTER I. The Natural History of the Common Crayfish • 1 CHAPTER II. The Physiology of the Common Crayfish. The Mechanism by which the Parts of the Living Engine are supplied with the materials necessary for their maintenance and growth • 46 CHAPTER III. The Physiology of the Common Crayfish. The Mechanism by which the Living Organism adjusts itself to surrounding conditions and reproduces itself • 87 CHAPTER IV. The Morphology of the Common Crayfish. The structure and the development of the individual • 137 CHAPTER V. The Comparative Morphology of the Crayfish. The structure and the development of the Crayfish compared with those of other living beings • 227 CHAPTER VI. 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