7O ZOOLOGY. 



the muscle fibres are attached, and the nature of the medium 

 which must be penetrated. Many aquatic forms, though free- 

 swimming in their early stages, may become attached and give 

 up the power of locomotion in the adult condition. Such 

 attached forms ordinarily secrete an external shell or covering 

 into which they can withdraw for protection (e. g., barnacles, 

 many polyps ) . They must depend upon currents in the water 

 to supply them with food. They are frequently able to produce 

 the currents by the motion of parts of the body. The majority 

 of active movers have hard parts which serve as levers to 

 which the muscles are attached. The parts of the skeleton, 

 which may be either external to the muscles or surrounded by 

 them, articulate with one another by a hinge or movable joint, 

 as illustrated by vertebrates or insects. In some forms with- 

 out a conspicuous skeleton, as the earthworm, there is a 

 dermo-muscular wall surrounding a fluid-filled cavity. By the 

 alternate use of the longitudinal and circular fibres, changing 

 the relative position of the parts of the body, locomotion is 

 effected. The special appendages, particularly the paired 

 appendages, are important motor organs in nearly all actively 

 moving animals. 



101. 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 environ- 

 ment to be among the most important experienced by the 

 organism, and that the superficial protoplasm by virtue of its 

 irritability (see also 19) 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. 



102. As an organism increases in the number and variety 

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



