138 ANIMAL LIFE 



as indirectly by the stimulus of sight, which is trans- 

 mitted to the skin by the nervous system. Moreover, 

 as the colour of the animal is deposited in the muscle- 

 pigments as much as, or even more than, in those of the 

 skin, and since the colour-changes of the muscles follow 

 those of the skin, it is evident that the eyes govern 

 changes in the muscle as well as those in the skin. 

 We are led to look upon the eyes, therefore, not only 

 as organs of sight, but rather as one link in a chain 

 of response which is evoked by light from the whole 

 animal. From this point of view the eyes, acting upon 

 the nervous system, both govern the colour-relations 

 of the shrimp and give the tone to its muscles and 

 general activity. 



In a somewhat similar way we may look upon the 

 response to vibration, the sense of pressure and of 

 equilibrium. The shrimp has two ' ears,' placed in 

 the first pair of horns, or in some shrimps near the 

 end of the tail. These ears are hollow balls containing 

 a solid body, either grains of sand or a small block of 

 lime ; and it has been found by experiment that 

 accurate balance during swimming or walking is de- 

 pendent upon the healthy condition of these ears. 

 Like the eyes, they communicate with the muscles 

 and skin by means of the nervous system and have 

 the faculty of compensating any tendency to upset or 

 overturn. They register every vibration that the 

 waves cause or that movement induces. These ears 

 are, in fact, organs of touch with a delicate balance 

 and intimate connection with the muscular system ; 



