186 



NERVOUS SYSTEM. 



axis of vertebrata, as that first shown by Lyonet in insects ; 

 the motor columns always lie above and in contact with the gan- 

 glionic, as shown by him in the cossus (Fig. 84. A. b, c.) All 

 the other viscera of the trunk present the same inverted po- 

 sition ; the heart-forming portion of the sanguiferous system 

 occupies the dorsal surface, and the great nervous columns, 

 the ventral surface of the alimentary canal, in the articulated 

 classes. The position of these organs and of the whole trunk, 

 and consequently the nervous columns, is reversed in the spini- 

 cerebrata, where the great centres of the nervous system are 

 inclosed in a distinct osseous sheath. 



In the lowest of the helminthoid classes, the entozoa, 

 where the animals remain, for the most part, permanently im- 

 bedded in the source of their nutrition, the nervous system 

 is very imperfectly developed, and is little required. The 

 more elevated forms of nematoid intestinal worms, as the 

 ascaris, (Fig. 82. A,) present a slender double, white, nervous 



FIG. 82. 



filament, occupying the median line of the abdomen, and 

 placed immediately within the inner longitudinal muscular 

 tunic. This abdominal nervous chord in the long cylindrical 

 body of the nematoid worms appears, from the close approx- 

 imation and the smallness of its component parts, to consist 



