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 orare 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-cesophageal 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-cesophageal 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 Physiologie du Syst¢me nerveux de 
?’Homme et des Animaux vertébrés.’ Paris, 8vo., 1842, tom. ii., p. 662. 
