2IO 



LANCDON. 



[Vol. XL 



Course of the nerves from the cetitral neiiioits system to the 

 epidermis. — As is well known, from each side of each ganglion 

 of the ventral nerve-chain three great nerves take origin. The 

 anterior pair of nerves arises just caudad of the anterior 

 septum of the metamere. The middle and posterior pairs, 

 which lie so closely together that they are often called " double 

 nerve-roots," arise just caudad of the middle of the ganglion 



t^ nr. 



%\T. miLi. 



3W.H 



iniWt.*--' 



latloK. {»»'' 



•yen. lui.U 



Fig. 2. A diagram showing the course of the median nerve-trunk and its formation of a nerve- 

 ring. This diagram holds good for the other nerve-trunks, except that the sense-organs would not 

 be so numerous as iu this. For reference letters, see text, Fig. i. 



(see text, Fig. i). All of these nerves pass latero-ventrad to the 

 inner surface of the longitudinal muscle-layer, and there divide 

 into a ventral and a dorsal ramus (see text, Fig. 2). These* 

 rami pass to the inner surface of the circular muscle-layer, the 

 ventral ramus using the space between the ventral and inner 

 intersetal tract of longitudinal muscles, the dorsal ramus using 

 the space between the inner intersetal and lateral tracts. On 

 reaching the circular muscle-fibres, the dorsal ramus sends a 

 very short branch ventrad between the circular and longitudi- 

 nal muscle-layers, and then turns and passes dorsad and slightly 

 caudad between the two muscle-layers. As the dorsal ramus 



