Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 139 



nately, as the muscles of the abdomen contract and air leaves the body, the 

 plates are drawn together with the edge of one overlapping the one behind it. 

 Insects and other arthropods also have hinge joints. The leg of a lobster or an 

 insect bends like a jackknife. 



Changing Content of Skeletons 



The content of skeletons is in part changeable, in part permanent. Their 

 composition depends upon the material brought by the blood to the cells which 

 produce the more rigid substance. What is brought depends upon the materials 



Fig. 9.4. Joints of the arthropod skeleton. 

 A, telescopic joints in the abdomen of an insect 

 when outstretched; pieces of skeleton held to- 

 gether by muscles and skin; B, insect's leg held 

 straight and flexed showing the stretching and 

 folding of the soft skin around the joints. (A, re- 

 drawn after Guyer: Animal Biology. New York, 

 Harper & Bros., 1936. B, redrawn after Ross: 

 A Textbook of Entomology. New York, John 

 Wiley & Sons, 1948.) 



in the animal's environment and the physiological pattern that the animal 

 inherits. 



Calcium, occurring in limestone, soil, and water, is continually passed in and 

 out of animals, but during its sojourn in an animal's body it is mainly located 

 in the skeleton. Striking exceptions are horny structures and the chitinous 

 skeletons of insects. In its usual state, 16 per cent of a crab's shell is calcium; 

 when it is "soft," such a shell is but one per cent calcium. This is the only 

 time when the shell stretches. 



The skeletons of primitive vertebrates are more or less cartilaginous; those 

 of vertebrate embryos are at first composed of cartilage, later mainly replaced 

 by bone. Cartilage is composed of connective tissue cells which produce a 

 more or less resilient gel. 



The connective tissue cells which produce bone form two different materials: 

 minerals, chiefly calcium and phosphorus, and collagen, a protein. The colla- 

 gen fibers are arranged spirally in the mineral matter, binding it like wires in 



