194 ARTHROPODA 



cord, dissect the esophagus from the nerve ring; free the 

 nerve cord from the cartilaginous bridges that extend across 

 it at intervals; sever the main nerves at some distance from 

 the brain and cord; lift the entire central nervous system out 

 and place it in a crystallization dish. 



Before this system is further studied it should be hardened 

 and whitened. First arrange the brain, cord and nerves in 

 a normal position and cover the system with a piece of dry 

 newspaper. Hold the paper in place with the fingers and 

 slowly pour over it enough 5 per cent formalin just to cover 

 the paper. In about half an hour the nervous tissue will be 

 hard and white. The preparation may then be turned to 

 expose the neural surface of the brain and cord, and the 

 origins of the neural nerves. 



Note the preoral commissure extending across the nerve 

 ring anterior to the position occupied by the esophagus. 

 The commissure gives off three rostral nerves directed 

 caudad. Find four postoral commissures, the last three of 

 which are fused. Just behind the forebrain arises a pair of 

 cheliceral nerves. Five pairs of large nerves are given off 

 from the nerve ring to the next five thoracic appendages. 

 Each of these gives off a neural mandibular branch and 

 several hemal entocoxal branches. Two pairs of small 

 nerves are given off from the posterior part of the nerve ring 

 at points neural to the origin of the nerve cord. These are 

 the chilarial and opercular nerves. 



The double nerve cord extends into the abdomen. Here 

 may be found 5 pairs of ganglia, and a terminal knot con- 

 sisting of 3 pairs of ganglia closely fused. Each of the first 

 five pairs of ganglia gives off a pair of hemal nerves directed 

 cephalad, and a pair of neural nerves directed caudad. 

 Usually the neural nerves are lacking in the last 3 pairs of 

 ganglia. The 3 pairs of hemal nerves are spread fan-wise. 

 The pair nearest the median line innervates the telson. 



Make a drawing of the nervous system from the neural 

 aspect. 



