96 CRABS AND INSECTS. 
from the stomach of the fresh-water cray-fish are used as an antacid, 
The horseshoe is valued as guano. 
Fic. 109.—Gigantic extinct crustacean (Prerygotus), nine feet long, swim- 
ming among Crinoids. (After Buckley.) 
Specimens for Study.—In dissecting a crab, cray-fish, 
or other specimen, the carapace should be carefully re- 
moved with knife or scissors, the gills examined, the mouth- 
parts removed and compared with cut No. 81, and the 
difference between the parts noted. The claws should 
also be compared, and their various offices thoroughly un- 
derstood. The brain, stomach, and muscles can be stud- 
ied by cutting away the red membranous hypodermis. 
By injecting carmine into the arteries through the heart, 
the arterial system can be traced. The eggs, eyes, etc., 
can be hardened in alcohol, and cut in sections for micro- 
