266 



Animal Life and Intelligence. 



au. 



Fig. 28.— Tail of Mysis. 



au., auditory organ. 



In the cuttle-fishes it is found embedded in the cartilage of 

 the head. 



In the lobster or crayfish the auditory organs are found 

 at the base of the smaller feelers or antennules. They are 

 little sacs formed by an infolding of the external integument 

 (see Fig. 26, p. 259). Beautifully feathered auditory hairs 

 project into the sac along specialized ridges, and the sac 

 in many cases contains grains of sand which play the part 

 of otoliths. Hensen seems to have proved that shrimps 

 collect the grains of sand and place them in the auditory 



sac for this purpose. The 

 curious shrimp-like Mysis has 

 two beautiful auditory sacs in 

 its tail. These are provided 

 with auditory hairs. Hensen 

 watched these under the micro- 

 scope while a musical scale was 

 sounded, and found that the special hairs responded each 

 to a certain note. When this particular note was sounded 

 the hair was thrown into such violent vibration as to become 

 invisible, but by other notes it was unaffected. 



Passing now to insects, we may first note that grass- 

 hoppers and crickets have an auditory organ on the front 

 leg. These are provided with tympanic mem- 

 branes, and the breathing-tubes, or tracheae, 

 are so arranged that the pressure of the air 

 is equalized on the two sides of the mem- 

 brane — just as in us and other vertebrates the 

 same end is effected by a tube which runs 

 from the interior of the drum of the ear to the 

 mouth-cavity (see Fig. 27). In the organ within 

 the leg there is a group of cells, followed by a 

 row of similar cells which diminish regularly in 

 size from above downwards. Each is in connection with a 

 nerve-fibril, and contains a delicate auditory rod. It has 

 been suggested that the diminution in size of the cells may 

 have reference to the appreciation of different notes, but 

 nothing definite is known on the matter. Ants, too, have 



Fig. 29.— Leg 

 of grasshopper. 



ty., tympanic mem- 

 brane. 



