INSECTA. 



199 



The air-breathing articulated animals with jointed legs offer a close 

 correspondence with those that respire by gills in the progressive steps 

 of complication of the nervous system and the order in which those 

 steps succeed each other. The lowest insects, like the lowest Crus- 

 taceans, resemble the worms, in the great length and slenderness of 

 their body, and in the uniform size, shape, and number of the con- 

 stituent segments. In the lulus, whose very short and numerous 

 rings support each two pairs of rudimental legs, the corresponding 

 ganglions of the abdominal chords are much less conspicuous than 

 in the earth-worms, and the whole central axis of the nervous 

 system, continued from the brain, is almost as devoid of partial 

 swellings as the spinal chord of the apodal vertebrate. It lies^ how- 

 ever, as in other Articulata, on the opposite side of the body to that 

 in which the brain is situated. 



The cephalic ganglion (Jig.98. a) of the lulus is transversely elon- 



lulus. 



gated, and obscurely divided by a slight median indentation into two 

 side-lobes : its upper and latter extremities are prolonged outwards 

 into the short and thick optic nerves (c, c), which resolve themselves 

 half way towards the compound eye into a plexus of filaments for its 

 several divisions. Two separate antennal nerves, conjectured by Straus 

 to be motory and sensory (d, d), are sent off on each side below 



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