CHAPTEE V 



THE SKELETON 



The skeletal tissues of the animal body show a variety which is at 

 first sight quite bewildering. Closer scrutiny however reveals certain 

 general principles which are at work. In a. very restricted set of 

 cases we see that the supporting structure consists of a row or rod of 

 cells which is rendered stiff through the individual cells being blown 

 out or distended with fluid. Such turgor of cells is a far less con- 

 spicuous feature in the animal kingdom than it is in the vegetable. 

 It is well seen in the axial row of endoderm cells which supports 

 the tentacles of the Hydrozoa. In the Vertebrate it is seen in the 

 notochord. 



Ear more usually the support is given by a definite supporting 

 substance with such physical qualities as rigidity, tensile strength, 

 elasticity, as may be required in the particular case. 



These supporting substances of the animal body again show the 

 greatest variety in their morphological nature but they may all be 

 classed between two extremes — in one of which the supporting 

 substance consists clearly of modified cells or portions of cells and in 

 the other of dead intercellular substance. Examples of the former 

 are seen in the remarkable phagocytic organs of nematode worms 

 where an enormous cell becomes developed into an immensely com- 

 plicated branched structure of stiff horny consistency upon the 

 terminal twigs of which are perched innumerable minute blobs of 

 phagocytic protoplasm. A good example of the second type is seen 

 in the skeleton of ordinary coral — a mass of hard calcareous material 

 lying clearly outside the limits of the living cells. 



It is necessary to emphasize the fact, which is frequently lost 

 sight of, that the differences between these two types are superficial 

 rather than fundamental. They are merely the extremes of a series 

 and are connected up by innumerable intermediate conditions. The 

 skeleton of an Arthropod such as a Lobster is in the early stages of its 

 development simply the stiffened and hardened outer layer of the 

 cytoplasm of the ectoderm cells, while in its latest stage, immediately 

 before it is shed, it has become a thick layer of dense chitinous 

 and calcified non-living substance lying outside the limits of the 

 living protoplasm. 



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