Norman, Nervous System of Arthropods. 237 



acic ganglia, appears to vary for different animals. Only Asta- 

 cus and Hydrophilus were more accurately studied as to this 

 point. For Hydrophilus the three thoracic ganglia are equally 

 coordinated centers, each being the center for the various re- 

 flex actions of the corresponding segment. On the other hand, 

 in the case of Astacus, when the ventral cord is cut through 

 between the first and second pair of walking legs there is a 

 greater loss than when the section is made just behind the 

 mouth ganglia ; there is not only the loss of progressive motion, 

 but also the movements of feeding. All other movements, par- 

 ticularly those of flexion and extension are localized in the 

 ganglia of their respective segments. 



6. Section of the transverse commissures between the 

 halves of one or more ganglia produces no paralytic effects; 

 for example, longitudinal section of the brain of Carcinus, Asta- 

 cus, Apis, Dytiscus ; similar sections of the thoracic ganglia of 

 Astacus and Carcinus. It follows from this, that the motor 

 elements for each half of the body lie in the corresponding 

 halves of the ganglia, or at least that all the motor impulses to a 

 muscle in no case make a total crossing in the ganglia. 



7. The influence which the brain or any part of the cen- 

 tral nervous system exercises upon the parts lying more poster- 

 iorly is conducted along one side only of the ventral cord, i. e., 

 no crossing through the transverse commissures. (If a longi- 

 tudinal commissure is cut through, the symptoms appear only 

 along one side of the animal — Astacus, Hydrophilus). 



8. A stimulus from the brain along the ventral cord is 

 conducted with a local signal, i. e. , so that a locally (or defi- 

 nitely) directed reaction follows, only through the oesophageal 

 commissure of the stimulated side. 



9. The transverse commissures of the brain can conduct 

 impulses to the longitudinal commissure of the other side and 

 thus to the ventral cord, but not in such a way as to bring 

 about a definite reflex action ; and even then the stimulus must 

 be very strong. 



10. It may be concluded from the experiments on the 

 crayfish (Astacus), which have been in part confirmed upon 



