NERVOUS AND SENSORY SYSTEMS. 259 



of the tail ; (2) the twisted ventral muscles, most of which 

 are flexors or benders of the tail, which have harder work, 

 and are much larger than their opponents ; (3) those mov- 

 ing the appendages ; (4) the bands which work the gastric 

 mill. 



Nervous System. 



The supra-oesophageal nerve centres or ganglia, forming 

 the brain, have been shunted far forward by the growth of 

 the pre-oral region. We thus understand how the nerve 

 ring round the gullet, connecting the brain with the ventral 

 chain of twelve paired ganglia, is so wide. 



The dorsal or supra-oesophageal ganglia are three lobed, 

 and give off nerves to eyes, antennules, antennae, and 

 food canal, besides the commissures to the sub-cesophageal 

 centres. 



The sub-cesophageal ganglia, the first and largest of the 

 ventral dozen, innervate the six pairs of appendages about 

 the mouth. There are other five ganglia in the thorax, and 

 six more in the abdomen. 



Though the ganglia of ^ach pair are in contact, the ventral 

 chain is double, and at one place, between the 4th and 5th 

 ganglia, an artery (sternal) passes between the two halves 

 of the cord. From each pair of ganglia nerves are given 

 off to appendages and muscles, and apart from the brain, 

 these minor centres are able to control the individual move- 

 ments of the limbs. In the thoracic region the cord is well 

 protected by the cuticular archway already referred to. 



From the brain, and from the commissure between it and 

 the sub-cesophageal ganglia, nerves are given off to the food 

 canal, forming a complex visceral or stomato-gastric system: 

 Similarly from the last ganglia of the ventral chain, nerves 

 go to the hind gut. If the brain be regarded as the fusion 

 of two pairs of ganglia, as the development suggests, and 

 the sub-cesophageal as composed of six fused pairs, then 

 these, along with the eleven other pairs of the ventral chain, 

 give a total of nineteen nerve centres, a pair for each pair 

 of appendages. 



Sensory System. 



A skin clothed with chitin is not likely to be in 

 itself very sensitive, but some of the setae are. These are 



