9 o 



ELEMENTARY ZOOLOGY. 



Locomotion in different animals differs very much in 

 its efficiency. This may depend on the character and 

 amount of the muscular tissue itself, or on the attachments 

 of the muscles, or on the satisfactory adjustments of 

 the hard parts which serve as levers on which the muscles 

 act, or, finally, on the nature of the medium in which the 

 organism habitually moves. 



While most animals have both motion and locomotion, 

 there are many attached forms which have only the power 

 of extending or withdrawing certain parts. Even in 

 such forms as these, however, the young usually have a 

 period of free locomotion before settling down. This 

 power is of value in scattering the species, and suggests 

 that the remote ancestors of even the fixed forms were 

 free-swimming. 



96. Practical Exercises for Library or Field. Make a study of the 

 special manner of locomotion in a series of animals in different 

 media air, water, or solids. What variety are you familiar with 

 in swimming organs? In flying organs? In burrowing organs? 

 In organs for walking on solids? Do any animals have satisfactory 

 locomotion without any apparent special organs of locomotion? 

 How is it done? 



97. Sensation and Sensory Structures. We have seen 

 that the simplest bit of protoplasm is sensitive. This 

 means that certain changes in the outside conditions 

 bring appreciable changes in the behavior of the proto- 

 plasm. Protoplasm is irritable. These external stimuli 

 first act at the surface of the simple cell ; but undoubtedly 

 the results pass on inward until all the internal structure 

 is changed in some degree, if we could only see it. In 

 other words, the protoplasm has the power of conducting 

 impulses from one part to another. A stimulus at one 

 side may produce its visible results at the other side. 



