THE SKELETON EXTERNAL AND CALCIFIED. 17 
care to go far beyond the surface of things, would find to 
notice in the animal itself. 
Probably the most conspicuous peculiarity of the cray- 
fish, to any one who is familiar only with the higher 
animals, is the fact that the hard parts of the body are 
outside and the soft parts inside; whereas in ourselves, 
and in the ordinary domestic animals, the hard parts, or 
bones, which constitute the skeleton, are inside, and the 
soft parts clothe them. Hence, while our hard framework 
is said to be an endoskeleton, or internal skeleton; that 
of the crayfish is termed an exoskeleton, or external 
skeleton. It is from the circumstance that the body of 
the crayfishes is enveloped in this hard crust, that 
the name of Crustacea is applied to them, along with 
the crabs, shrimps, and other such animals. Insects, 
spiders, and centipedes have also a hard exoskeleton, 
but it is usually not so hard and thick as in the 
Crustacea. : 
If a piece of the crayfish’s skeleton is placed in strong 
vinegar, abundant bubbles of carbonic acid gas are given 
off from it, and it rapidly becomes converted into a soft 
laminated membrane, while the solution will be found to 
contain lime. In fact the exoskeleton is composed of 
a peculiar animal matter, so much impregnated with 
carbonate and phosphate of lime that it becomes dense 
and hard. 
It will be observed that the body of the crayfish is 
naturally marked out into several distinct regions. ‘There 
c 
