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 exoskeletoa, 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 bod}'^ of the crayfish is 

 naturally marked out into several distinct regions. There 



