124 INVERTEBRATE ZOOLOGY. 



97. Nervotis system. This consists of two portions which are more con- 

 veniently studied in separate dissections. 



1. The ventral nerve cord. This lies in the centre of the ventral side, and 

 is found in the floor of the trough made by the body walls, after removal of 

 the alimentary canal and reproductive organs. Prepare a fresh specimen by 

 removal of these parts, or use one from which they have already been taken. 

 Wash carefully in 80 ',r and i)lace in 95 ^c. Strong alcohol whitens and hardens 

 nervous matter. The nerve cord is easiest foiind in the abdomen, where it is 

 only slightly protected by internal prolongations of the skeleton. In the tho- 

 rax the cord is more carefully protected and is best dissected by tracing it up 

 from the abdomen and removing the overlying parts. The cord 'W'ill be found 

 to consist of rounded bunches, the ganglia or nerve centers, connected by a 

 median nerve cord which is in reality double, as may be proven by removing 

 its sheath with a needle. From the ganglia fine nerves pass out to the sur- 

 rounding parts. How many abdominal ganglia are there ? How many tho- 

 racic ? Typically there is a ganglion to each segment, but in higher forms 

 there is more or less consolidation, especially at the end of the abdomen 

 where there is a modification of the original segments. Draw. 



2. The " brain'' {supra -oesophageal ganglion) and the other structures of 

 the head. This is more delicate than any of the previous preparations, but is 

 done in the same manner. Remove the head from a large specimen, and, 

 holding it between the thumb and finger, shave off the siirface of the face 

 with a sharp scalpel. Place it in the dissecting pan. cover with 95^ and ex- 

 amine with the lens. Fine pins may be put through the extreme edge, to 

 hold it in place. A whitish mass will be imperfectly seen throiTgh the irregu- 

 lar hole made by the scalpel. This is the brain or supra-oesophageal ganglion, 

 and may be brought into full view by cutting away the edges of the hole and 

 removing a mass of loose fat which often covers it. Notice the huge optic 

 lobes which go to the eyes, a pair of solid cups connected by stalks ; between 

 these a pair of ocellar lobes with nerves to the lateral ocelli ; beneath, there 

 are a pair of antennal lobes, supplj'ing nerves which nan through the anten- 

 nae. In the center of the mass may be seen a short bit of nerve cut off from 

 its connection by the removal of the facial wall. This is the nerve to the me- 

 dian ocellus. Draw the brain. The brain lies dorsal to the alimentary canal 

 and connects with the remainder of the nervous system by a pair of large 

 nerves or co))iniissa7-es which are found at the ventral lateral corners of the 

 brain, behind the antennal lobes. They may be followed around the oesoph- 

 agus, where they unite with the infra-oesophageal ganglion, the first of the 



