THE MODIFICATION OF ORGANS AND FUNCTIONS 27 



posed of a series of body-rings or segments; the toad's 

 body is a compact apparently unsegmented mass. The 

 toad has eyes each with a single large lens and capable 

 of moving in the head and of changing their shape and 

 hence their focus; the crayfish's eyes are immovable and 

 have a fixed focus, and are composed of hundreds of tiny 

 eyes each with lens and special retina of its own. And 

 so a long list of differences might be gone through with. 



Resemblances between toad and crayfish. But on the 

 other hand there are many resemblances resemblances 

 both in structure and life-processes or physiology. Both 

 toad and crayfish have organs for the prehension of food, 

 its digestion and its assimilation. And these organs, the 

 organs of the digestive system, while differing in details 

 are alike in being composed principally of a long tube, 

 the alimentary canal, running through the body, open 

 anteriorly for the taking in of food, and open posteriorly 

 for the discharge of indigestible useless matter. Both 

 alimentary canals are divided into various special regions 

 for the performance of the various special processes con- 

 nected with the digestion and assimilation of food. Each 

 is adapted for the special kind of food which it is the habit 

 of the particular animal to take. The two sets of organs 

 are essentially alike and have the same essential function 

 or life-process to perform. But this process differs in the 

 details of its performance, and the organs which perform 

 this function and which constitute the digestive system of 

 each are modified to suit the special habits or kind of life 

 of the animal. 



Both toad and crayfish have a heart w^ith blood-vessels 

 leading from it. In the case of the toad the heart is more 

 complex than in the crayfish, and the system of blood- 

 vessels is far more extensive and elaborate. But the heart 

 and blood-vessels in both animals subserve the same pur- 

 pose; their function is the circulation of the blood, this 



