808 ANIMAL LIFE 



is present in fish, salamanders, or snakes, which fact ac- 

 counts in part for their partial deafness. 



More simple auditory organs are found among some of 

 the invertebrates. These consist of small sacs containing 

 a clear liquid with one or more tiny lime nodules, called 

 otoliths. Specialized cells often bearing vibratile hairs 

 line more or less of the inner surface of the sac. Organs 

 of hearing or equilibration are present in the margin of the 

 umbrella of certain jellyfish, in the head of a few worms, 

 in the foot of the mussel, in the head of the snail, in the 

 first segment of the antennules of the crayfish, and in some 

 other crustaceans in the tail. Hearing among insects is in 

 many species quite acute, and it is probable that they are 

 aware of some sounds that make no impression on our 

 ears. In a few insects a vibrating tympanic membrane 

 occurs as may be seen in the common locust, having the 

 ear on the side of the first abdominal segment. The true 

 grasshoppers and crickets have an ear on the front leg. The 

 oval tympanic membrane may be seen with the naked eye 

 on the outer surface of the tibia of the common large 

 cricket. It has the same location in ants. Certain other 

 insects possess a complex organ of hearing at the base of 

 each antenna. 



Sight is undoubtedly the most important sense of all for 

 animals living in the light, because it not only aids them 

 greatly in securing food, but also in escaping from their 

 enemies. All vertebrates have eyes or rudiments of them. 

 Moles living underground, insects, crustaceans, fish, and 

 amphibians inhabiting caves, and some fish living at great 

 depth in the sea have eyes of such a rudimentary char- 

 acter as to be of no use. The larger mammals can sight 



