408 MR. NEWPORT ON THE NERVOUS SYSTEM OF THE SPHINX LIGUSTRI. 



in the expulsion of faeces, and the middle lamella of the tail, in which the anus is situ- 

 ated. The remaining pairs of nerves (w) are given to the other lamellae of the tail. 



All the nerves I have now described in the Lobster belong to what Sir Charles 

 Bell calls the regular or symmetrical, and come directly from ganglia ; but there are 

 others [fig. 40. (q, q, 9)], which come directly from the upper surface of the cords, 

 unconnected with those from ganglia. In the caudal region there are two sets of these 

 posterior to each ganglion. They arise from the tracts by single trunks, each dividing 

 into five or six branches, that ramify in every direction, and are given entirely to the 

 muscles. Although at first sight they appear to form ganglionic enlargements (q, q) 

 before dividing into branches, there are no ganglia upon them. This appearance is 

 occasioned by the approximated fibres which constitute the trunk being spread out, 

 instead of rounded like a cord. The two last of these nerves (r) originate singly 

 from the tracts, and are given to the under surface of the rectum. In the thoracic 

 region they come from the tracts [Plate XVII. figg. 40. and 42. (/, /)] immediately 

 above the posterior part of the ganglia, and are given to the muscles of the branchiae. 



The detection of a double spinal column in the Lobster has since led me to ex- 

 amine more closely the nervous system of the Scorpion, one of the Aracknida (Scorpio 

 europceus, Linn.). Upon showing my dissection of the Lobster to Professor Grant, he 

 directed my attention to a structure observed in the Scorpion by Professor Muller, 

 of Bonn, which has been thought to be the motor tract. This structure I had not at 

 that time observed. It consists of a straight narrow slip, or riband, extending along 

 the median line, above the cords and ganglia, from the great thoracic mass, over 

 which it is expanded, to the last caudal ganglion, and is nearly of uniform size 

 through its whole length. It is connected by some exceedingly small fibres with 

 the nerves, while passing over the ganglia, and is, I believe, analogous to the trans- 

 verse or involuntary nerves of insects. The true motor tract appears to be closely 

 adherent to the sensitive in the Scorpion, the same as in the Lobster, and is scarcely 

 observable even where it passes over a ganglion. It is nearly equal in size to the 

 sensitive, with which it is connected. The nerves given off unite with those from the 

 ganglia, j ust the same as in the Lobster. 



The double structure of the nervous cords is more distinctly seen in one of the 

 Myriapoda {Scolopendra morsitans, Linn.) [Plate XVII. fig. 43.] than in the Scorpion. 

 The two longitudinal cords are united by twenty-three ganglia, and are composed of 

 two tracts [figg. 44. and 45. (a, h)'], lying one over the other, as in the Lobster and 

 Scorpion. The ganglia are entirely on the under surface of the cords [Plate XVII, 

 fig. 47. («)], and the existence of the motor tract is very evident after it has been for 

 some time in alcohol. It is marked by a line [fig. 47. {d)~\ which passes laterally 

 over the ganglia, and is continued along the lateral surface of the cords. The nerves 

 from the motor tract come off as filaments, anterior to those from the ganglion, with 

 which they immediately unite. Four pairs of nerves are given off from each ganglion, 

 and a fifth pair passes oflf from the motor tract [fig. 46. (e)] immediately posterior 



