THE NERVOUS SYSTEM OF THE LOBSTER. 315 



nerves on each side of considerable size which pass to the limb of the 

 corresponding side. Besides these there are other smaller nerves dis- 

 tributed to the wall of the thorax, but these do not concern the pre- 

 sent question. Of these two nerves the one is anterior and the other 

 posterior with regard to the normal position of the animal : but both 

 nerves arise in the same horizontal plane, i.e. one is not dorsal to the 

 other. The anterior nerve is much the smaller in the great chelae and 

 first and second ambulatory legs, where the posterior nerve is of great 

 thickness. In the last two ambulatory legs the two nerves are nearly 

 the same size. Throughout this paper the anterior will be spoken of 

 as the "small nerve" and the posterior as the "large nerve." 



On tracing these two nerves to their distribution in the large chelae 

 in specimens hardened in alcohol, the small nerve seemed to consist of 

 two parts, one of which could be traced to the divaricator muscle (ad- 

 ductor of Huxley x ) which opens the claw. The other part of the nerve 

 appeared to go to the skin. But no such division into two parts was 

 found in the fresh specimen. The small nerve also supplies the 

 extensor muscles of the various joints of the limb. 



The large nerve gives off branches to the flexor muscles of the limb, 

 but is chiefly distributed to the large occlusor muscle (adductor of 

 Huxley 1 ) which closes the claw. Also there are numerous branches 

 to the skin of the claw. 



In the case of the first or sub-oesophageal ganglion the nerves are so 

 crowded together that it is difficult to determine whether the nerves 

 to the masticatory appendages which arise from the ganglion are simi- 

 larly arranged. 



2. Abdominal Nerves. — From each abdominal ganglion two nerves 

 arise on each side in a similar manner to the nerves of the thorax. 

 The anterior of these is distributed to the abdominal appendage of the 

 same side, and in specimens hardened in alcohol appeared to consist 

 of two parts, but no such division was found in fresh specimens. 



The posterior nerve, after passing between the abdominal muscles, 

 terminates in the skin along the side of the abdomen. 



The abdominal muscles themselves are supplied by nerves which 

 arise chiefly from the abdominal nerve cord between the ganglia : 

 but according to Yung the superior (posterior) nerve ramifies in the 

 muscle of the abdomen. 2 



1 Huxley : The Crayfish, p. 93. 

 a toe. cit., p. 487. 



