INSECTA. 



357 



afterwards the more numerous minute nerves to the little feet, which, 

 by their articulation to the segments in double pairs, indicate such 

 segments to be severally a confluence of two. The simplicity of the 

 abdominal chords corresponds with the close approximation and 

 great numbers of the organs from which they receive impressions 

 and to which they transmit stimuli. The analogy of this exceptional 

 condition of the abdominal chord or nervous axis in the lulus to the 

 dorsal spinal chord of the Vertebrata, is as instructive as is that of 

 the equally exceptional ganglionic condition of the spinal chord in 

 the Tetrodon amongst fishes to the normal abdominal knotted chords 

 in the Articulata, in tracing their mutual relations to each other. 



The segments of the Polydesmus are relatively fewer and larger 

 than in the lulus, and their lateral margins are produced: each, 

 however, with the exception of the first three, which answer to the 

 thorax in hexapod insects, supports two pairs of legs : but these are 

 longer than in the lulus. Accordingly we find the sub -abdominal 

 nervous chords {fig. 145, z), which show as little trace of their median 

 separation as in the luli, swelling into two slight enlargements (^, ^) 

 opposite each of the abdominal segments : two nerves are sent off 

 from either side of each enlargement, and the anterior of these four 

 pairs of nerves is directed at an acute angle forwards and outwards 

 to the stigmata: the remaining pairs supply the muscles 145 

 of the segment and the legs, and are of equal size. 



In the Centipede, a series of equal and equidistant \ 

 ganglia is developed upon the ventral surface of the two \ 

 abdominal chords. Only in the first and last of the ab- 

 dominal ganglions can any modification of size be de- ^^^ 

 tected. The anterior, or sub- oesophageal ganglion, for ;^ 

 example, is larger than the rest, having to supply the ^ 

 modified legs which perform the function of jaws and 

 undePlip ; the chords, diverging as they escape on each ;; 



side of the oesophagus, enclose it by uniting with the . 

 large bilobed ganglion, or brain above. The nerves from \, 

 this part supply the large antennae and the aggregated 

 ocelli. In the structure of the abdominal columns a tract 

 less closely connected with the ganglionic nerves may 

 be traced along their dorsal aspect. This was first pointed 

 out by Mr. Newport, who attributes to it the motor 

 function. A large, vascular trunk, connected also with 

 the dorsal aspect of the nervous system, has been regarded as part 

 of the nervous system ; by some as a motor, by others as a respiratory 

 column : its true nature was detected by Mr. Lord.* AYith regard to 



^ 



Polydesmus. 



* Med. Gazette, March 3d, 1838. 



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