NOTES. 



NOTE 1., CHAPTER L, p. 17. 

 THE CHEMICAL COMPOSITION OF TB1C EXOSKELETOff. 



T HE harder parts of the exoskeleton of the crayfish contain rathei 

 m- re than half their weight of calcareous salts. Of these nearly 

 seven-eighths consist of carbonate of lime, the rest being phosphate ol 

 litre. 



The animal matter consists for the most part of a peculiar substance 

 termed Chitin, which enters into the composition of the hard parts not 

 only of the Arthropoda in general but of many other invertebrated 

 animals. Chitin is not dissolved even by hot caustic alkalies, whence 

 the use <>f solutions of caustic potash and soda in cleaning the skeletons 

 of crayfishes. It is soluble in cold concentrated hydrochloric acid with- 

 out change, and may be precipitated from its solution by the addition of 

 water. 



Chitin contains nitrogen, and according to the latest investigations 

 (Ledderhose, "Ueber Chitin und seine Spaltungs-produkte : " Zeitschrift 

 fiir Physiologische Chemie, II. 1879) its composition is represented by the 

 formula C lf H. M N, O 10 . 



NOTE II., CHAPTER L, p. 29. 

 THE CRIB'S EYES, OR GASTROLITHS. 



The " Gastroliths," as the " crab's eyes " may be termed, are found 

 folly developed only in the latter part of the summer season, just before 

 ccdysis sets in. They then give rise to rounded prominences, one on 



