CHAPTER V 



THE MODIFICATION OF ORGANS AND FUNC- 

 TIONS 



Differences between crayfish and toad. In the dis- 

 section of the crayfish one of the most important things in 

 the study of zoology has been learned. It is plain that 

 the crayfish has a body composed, like the toad's, of 

 parts or organs, and that most of these organs, although 

 differing much in appearance and actual structure from 

 those of the toad, correspond to similarly named organs 

 of the toad, and perform the same functions or processes, 

 although with many striking differences, essentially in the 

 same way as in the toad. But the structure of the body 

 is very different in the two animals. The toad has an 

 internal body skeleton to which the muscles are attached, 

 and a soft, yielding, outer body-covering or skin ; the 

 crayfish has no internal skeleton, but has its body covered 

 by a horny, firm body-wall to which the muscles are 

 attached. The toad has its main nervous chain lying just 

 beneath the dorsal wall of the body; the crayfish has its 

 main nervous chain lying just above the ventral wall of 

 the body. The toad has lungs and takes up oxygen from 

 the air of the atmosphere ; the crayfish has gills and takes 

 up oxygen from the air which is mixed with the water. 

 The toad has a single pair of jaws; the crayfish has 

 several pairs of mouth-parts. The toad has four legs 

 fitted for leaping; the crayfish has numerous legs fitted 

 for crawling or swimming. The crayfish's body is com- 



26 



