328 THE MAINTENANCE OF TFIE INDIVIDUAL 



All of these muscles are under the control of the nervous system, while 

 energy for their continued movement must be furnished by means of 

 absorbed food transported through the circulatory system to every 

 part of the body. To visualize this inter-relationship think of the 

 sustained movement of an arm or leg which is dependent upon the 

 activity of numerous muscles. The action of the muscles is in turn 

 controlled by the nerves which conduct messages to the tissues from 

 the brain and spinal cord. The entire network of nerves and their 

 branches has often been likened to a telephone system with its compli- 

 cated series of connections and relay wires. Closely associated with 

 the nerves are the arteries and veins, forming the triumvirate so 

 often pictured in histological or medical texts. 



The Kinds of Skeletons 



Skeletal support is of common occurrence in the animal kingdom. 

 Skeletons may be divided typically into outer coverings, or exo- 

 skeletons, and inner supporting devices, or endo skeletons. 



Exoskeletons 



Generally speaking, any creature or organism possessing 07ily an 

 exoskeleton belongs to the large group of invertebrate, or non-chordate, 

 animals. Such forms may be present in some members of a given 

 phylum and not in others. Even in the protozoa, for example, the 

 shelled arcellidae occur in the same class with the naked Ameba. 

 Other examples within this same group are the foraminifera and 

 radiolaria which possess limy or glassy skeletons. This suggests that 

 on the whole these types of exoskeleton are not essential for loco- 

 motion but are primarily protective devices. That is certainly true of 

 the sessile sponges, corals, sea-lilies, and lamp-shells (brachiopoda), 

 and would also probably hold for most of the clams, snails, star- 

 fishes, and brittle-stars. In the great phylum of the arthropods, the 

 exoskeleton is specialized and definitely associated with an equally 

 highly adapted muscular system, the two being definitely designed for 

 effecting locomotion. Even among the vertebrate chordates an 

 exoskeleton as well as an endoskeleton sometimes occurs, as, for 

 example, in the turtles. In such forms the vertebral column becomes 

 fused to the dorsal shell which is formed by the flattened ribs plus 

 dermal costal plates. 



