It will be observed that these differences of opinion turn upon questions of grouping and nomenclature. It would make no difference to the general argument if it were admitted that the whole body consists of twenty-one somites and the head of seven. NOTE XV., CHAPTER IV., p. 199. THE HISTOLOGY OF THE CRAYFISH. In dealing with the histology of the crayfish I have been obliged to content myself with stating the facts as they appear to me. The discussion of the interpretations put upon these facts by other observers, especially in the case of those tissues, such as muscle, on which there is as yet no complete agreement even as to matters of observation, would require a whole treatise to itself. {356} NOTE XVI., CHAPTER IV., p. 221. THE DEVELOPMENT OF THE CRAYFISH. The remark made in the last note applies still more strongly to the history of the development of the crayfish. Notwithstanding the masterly memoir of Rathke, which constitutes the foundation of all our knowledge on this subject; the subsequent investigations of Lereboullet; and the still more recent careful and exhaustive works of Reichenbach and Bobretsky, a great many points require further investigation. In all its most important features I have reason to believe that the account of the process of development given in the text, is correct. NOTE XVII., CHAPTER VI., p. 297. PARASITES OF CRAYFISHES. In France and Germany crayfishes (apparently, however, only _A. nobilis_) are infested by parasites, belonging to the genus _Branchiobdella_. These are minute, flattened, vermiform animals, somewhat like small leeches, from one-half to one-third of an inch in length, which attach themselves to the under side of the abdomen (_B. parasitica_), or to the gills (_B. astaci_), and live on the blood and on the eggs of the crayfish. A full account of this parasite, with reference to the literature of the subject, is given by Dormer (“Ueber die Gattung Branchiobdella:” Zeitschrift für Wiss. Zoologie, XV. 1865). According to Gay, a similar parasite is found on the Chilian crayfish. I have never met with it on the English crayfish. The Lobster has a somewhat similar parasite, _Histriobdella_. Girard, in the paper cited in the Bibliography, gives a curious account of the manner in which the little lamellibranchiate mollusk, _Cyclas fontinalis_, shuts the ends of the ambulatory limbs of crayfishes which inhabit the same waters, between its valves, so that the crayfish resembles a cat in walnut shells, and the pinched ends of the limbs become eroded and mutilated. {357} BIBLIOGRAPHY. The subjoined list indicates the chief books and memoirs, in addition to those mentioned in the text and in the Appendix, which may be advantageously consulted by any one who wishes to study more fully the biology of the crayfishes. I.—NATURAL HISTORY. ROESEL VON ROSENHOF. Der Monatlich-herausgegeben Insekten Belustigung. 1755. CARBONNIER. L’Écrevisse, Paris, 1869.