CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 131 



right and left sides of the guUet and forms the nerve collar. 

 The cords afterward join and become closely fused and 

 studtled with ganglia in the thorax and abdomen. Many 

 of the ganglia have become fused so that there are only 

 thirteen distinct ganglia for the twenty body segments. 

 From the ganglia nerves pass to all the organs of the body. 



Senses of the crayfish. — The crayfish has the sense of 

 sight, touch, smell, and perhaps taste. 



Each of the two large eyes, which are compound, is made 

 up of a numljer of small, square areas, or facets, and is 

 borne on the end of a movable stalk. The stalks are ex- 

 tensiljle and can protrude the eyes or withdraw them out 

 of the wa}' of danger. In addition, each stalk "is muscular 

 and capable of turning the eye, when protruded, to look in 

 any direction." 



There are a number of stiff hairs, or setas, on the external 

 branch of each antennule that are supposed to be the seat 

 of the sense of smell. 



There is also a sac in the basal segment of each antennule 

 that is m free communication with the surroimding water. 

 Formerly these were supposed to he organs of hearing but 

 are now known to be balancing organs. These aid the 

 animal in maintaining its equilibrium. 



The sense of touch is widely distributed over the body, 

 although through such a thick crust it is probably not very 

 acute. The antenna;, however, are delicate organs of touch. 



Respiration of the crayfish. — The plumehke gills are 

 the organs of respiration. In the filaments of the gills 

 the blood is separated from the surroun(Ung water by a 

 very thin membrane and the carbon dioxide readily passes 

 through this tissue by osmosis and the oxygen is as rearlily 

 absorbed from the water in exchange. Tlie gill scoops 



