ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



191 



maining thoracic ganglia and the abdominal ganglia remain distinct in 

 each segment. 



The sense organs include a pair of stalked compound eyes on the 

 first body segment. The eyes are called compound because each is com- 

 posed of many individual eyes ; there are about 2,500 single eyes in each. 

 Each single eye is a long, cone-shaped structure. At the outside there 

 is a facet which serves as a lens, and internally there are light sensitive 

 cells which receive the image. Also, there are pigmented cells within the 

 eye. These cells contain colored granules which can move up and down 

 in the cell. This movement regulates the amount of light which reaches 

 the sensory cells. A similar function is performed by the iris of our 

 eyes. It opens and closes to accommodate different intensities of light. 



■ Circumesophageal 

 connectives 



Nerve cord 



Abdominal 



Brain 



Thoracic gang/ion 



Subesophageal ganglion 



Fig. 13.7. Dorsal view of the central nervous system of the crayfish. This diagram 



shows the appearance of the nerve cord and the ganglia on the ventral surface of the 



body after the body organs have been removed. 



Sense organs of equilibrium are found in the base of the antennules. 

 One of these consists of a small sac which contains fine grains of sand 

 which shift around as the crayfish changes its position. These shifting 

 grains stimulate delicate nerve endings in the sac and the crayfish is able 

 to adjust its position so as to maintain its equilibrium. One research 

 worker confused the crayfish by substituting iron filings for the sand 

 grains in these organs. When a magnet, placed above the animal, 

 caused the iron filings to be drawn to the top of the sacs, the crayfish 

 became disoriented and turned over on its back and remained there as 

 long as the magnet was working. 



The appendages and other parts of the body bear small sensory 

 bristles which are of two kinds. One group is sensitive to touch, while 

 another is sensitive to chemicals and might be called sense organs of 

 taste. 



