326 



PHYLUM ARTHROPODA 



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. They act as a true brain. 



The sub - oesophageal 

 ganglia, the first and largest 

 of the ventral dozen, inner- 

 vate the six pairs of appen- 

 dages about the mouth. 

 There are other five ganglia 

 in the thorax, and six more 

 in the abdomen. 



Though the ganglia of 

 each pair are in contact, 

 the ventral chain is double, 

 and at one place, between 

 the fourth and fifth 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 movements 

 of the limbs. In the 

 thoracic region the cord is 

 well protected by the cuti- 



FiG. 173. — Section of compound eye 

 of My sis vulgaris. — After Gren- 

 acher. 



m., Muscle of eye-stalk ; 1-4, ganglionic 

 swellings in the course of the optic 

 nerve ; n., the nerve librUs passing up 

 to the retinul« ; rh., the rhabdoms ; 

 re., elements of retinulae ; p., band of 

 pigment; c, crystalline cones; co., the 

 corneal facets with the subjacent nuclei. 



cular archway already referred to 



From the brain, and from the commissure between it and the sub- 

 oesophageal 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 develop- 

 ment suggests, and the sub-oesophageal 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. 



