76 ZOOLOGY 



movable joint, as illustrated by vertebrates or insects. In 

 some forms without a conspicuous skeleton, as the earthworm, 

 there is a der mo-muscular wall surrounding a fluid-filled cavity. 

 Locomotion is effected in these by the alternate use of the 

 longitudinal and circular fibres, changing the relative position 

 of the parts of the body. The special appendages, particularly 

 the paired appendages, are important motor organs in nearly all 

 actively moving animals. 



104. Sensation and Sensory Structures. In a simple bit 

 of protoplasm it is manifest that the differences between the 

 living matter and the outside world are greater than the struc- 

 tural differences between the parts of the protoplasm itself. 

 Thus we would expect the stimuli arising from the action of 

 environment upon the living material to be among the most 

 important experienced by the organism, and that the superficial 

 protoplasm by virtue of its irritability (see also 20) would most 

 promptly feel and respond to such stimuli. The changes thus 

 instituted will be felt sooner or later to the remotest parts of the 

 cell mass. This transfer of the effects of a stimulus through a 

 longer or shorter distance introduces us to a second nervous 

 function, internal irritability of protoplasm or conductivity. 



105. As an organism increases in the number and variety 

 of its cells, the specialized structures need to be more com- 

 pletely bound to one another. It becomes necessary not only 

 that they receive impulses from such parts as are favorably 

 situated for the reception of stimuli, but that a degree of co- 

 ordination of the interrelated parts be secured, in order that 

 just such response shall be made as will best meet the needs 

 of the organism. This power of coordinated response to ex- 

 ternal stimuli makes it possible for an organism to become suited 

 to its environment. 



1 06. In the higher forms, the work above described de- 

 mands five classes of structures (see Fig. 36) : (i) end or sense 

 organs, which are specially sensitive to stimuli of different orders, 

 as mechanical (touch), chemical, ethereal (light), etc.; (2) con- 

 ductive tracts (afferent nerves'), which connect (i) with (3) cen- 



