CHAPTER 1 5 



SUPPORT AND MOVEMENT 



Animals have devised many forms of tial support but are highly protective as 

 support for their bodies. The various struc- well. Vertebrates, on the other hand, have 

 tures not only hold the body together but adopted an internal skeleton designed pri- 

 also, in many cases, have an important pro- marily for support. It affords very little 

 tective function. To be sure, such single- protection to the soft external parts of the 

 celled animals as amoeba exist completely animal, although it provides excellent pro- 

 naked, with no protective covering what- tection for such vital organs as the brain, 

 ever. Their close relatives, however, such as heart, and lungs. 



Difflugia (Fig. 15-1), secrete a substance Let us consider the human skeleton as an 



which collects tiny siliceous particles example of a vertebrate skeleton, 

 (sand) and cements them together to pro- 

 vide an enclosure into which they may THE 

 draw themselves when hard pressed. Para- 

 mecium possesses a semi-rigid pellicle 



which gives it some external support so that The skeleton of man is similar to that of 



its body maintains a relatively constant other mammals, almost bone for bone, but 



shape. Sponges produce minute angular certain parts are emphasized more or less 



spicules which afford a rather rigid skele- than similar parts in other mammals. This 



ton, and many coral animals secrete sub- is because of the upright position his body 



stantial external skeletons. Larger inverte- has taken. All animals that have taken to 



brates, such as arthropods and mollusks, bipedal locomotion, the dinosaurs, the birds, 



provide themselves with hard outer cover- and the kangaroo, for example, have shifted 



ings which not only lend the body substan- their body weight so that some parts of the 



374 



SKELETON SYSTEM 

 IN MAN 



