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 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 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 
fiir Physiologische Chemie, II. 1879) its composition is represented by the 
formula C,, Hy. N, O,. 
NotE IL, 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 
