PHYSIOLOGY OF NERVOUS SYSTEM IN ARTHROPODS. 475 



we must rest contented with such information as has been 

 obtained from Arthropods generally. 



1. The ganglia of the ventral chain are reflex centres of the 

 segment to which the nerves given off by them are distributed. 

 The extent and character of the reflexes which can be obtained 

 after isolation of the ganglia, by division of the ventral nerve 

 cord, varies greatly in different groups. In the Crustacea, 

 according to Yung, reflexes are more marked in the posterior 

 abdominal than in the thoracic region. Movements of the 

 anus, the posterior part of the intestine and of the generative 

 armature can be readily produced after division of the ventral 

 cord ; all other reflexes obtained under the same conditions 

 are inco-ordinate, and are often either absent or are not readily 

 excited. Longet* stated that section of the ventral cord in 

 insects produces paralysis behind the point of section, and 

 concluded from this fact that the ganglia have no independent 

 action ; but Longet's statement appears too sweeping. Stimu- 

 lation of the isolated ganglia always produces reflexes, although 

 exhaustion of all nervous power appears to follow section very 

 rapidly ; probably the result of rapid degeneration of the con- 

 ducting tracts. 



2. The infra-resophageal ganglia are, so far as function is 

 concerned, similar to the ganglia of the ventral chain, and are 

 the reflex centres of the segments and segmental appendages 

 of the head, at least in the more generalised forms. Faivre 

 found in Dytiscus that they are also concerned in co-ordinating 

 the movements of the thoracic appendages; but this is certainly 

 not so in the Diptera, in which the co-ordinating centres are 

 situated in the thoracic ganglion, nor are reflexes excited by 

 stimulation of the proboscis of the decapitated fly. 



3. The supra-ossophageal centres are the sole seat of the 

 determination of voluntary acts (co-ordinate spontaneous 

 movements). When these ganglia are separated from the 

 ventral chain, co-ordinate reflex actions may be excited, but no 

 co-ordinate spontaneous acts occur. 



* Longet, F. A. ' Anatomie et Physiologic du Systeme nerveux de 

 1'Homme et des Animaux vertdbrds.' Paris, 8vo., 1842, torn, ii., p. 662. 



