NOTES. 



Note 1., Chapter I., p. 17. 

 THE CHEMICAL COMPOSITION OF THE EXOSKELETON. 



The harder parts of the exoskeleton of the crayfish contain rather 

 more than half their weight of calcareous salts. Of these nearly 

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

 lime. 



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

 termed Ghitin, 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 of 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 

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

 formula C„ H 20 N a 10 . 



Note II., Chapter I., p. 29. 



THE CRAB'S EYES, OR GASTROLITHS. 



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

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

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



