308 



FUNCTIONS OF THE CORD. 



Fig. 149. 



kingdom articulata exhibits a remarkable uniformity throughout the 

 whole series, while its character gradually becomes more elevated as we 

 trace it from the lowest to the highest diyisions of the group. It usu- 

 ally consists of a double nervous cord studded .with ganglia at intervals, 

 and the more alike the different segments, the more equal are these gan- 

 glia. The two filaments of the nervous cord are sometimes 

 at a considerable distance from one another, and the ganglia 

 are distinct, but more frequently they are in close apposition, 

 and their ganglia appear single and common to both. That 

 which may seem as the typical conformation of the nervous 

 system of this group is seen in the ganglionic cord of scolo- 

 pendra, or in that of the larvae of most insects, such as that 

 of the sphinx ligustri, Fig. 149. Here we see the nervous 

 cord nearly uniform throughout, its two halves being sepa- 

 rated, however, in the anterior portion of the body. The 

 ganglia are disposed at tolerably regular intervals, are simi- 

 lar to each other in size (with the exception of the last, 

 which is formed by the coalescence of two), and every one 

 supplies its own segment, and has little connection with any 

 other. The two filaments of the cord diverge behind the 

 head to inclose the oesophagus, above which we find a pair 

 of ganglia that receive the nerves of the eyes and antennas. 

 We shall find that in the higher classes the inequality in the 

 formation and office of the different segments, and the in- 

 creased powers, of special sensation, involve a considerable 

 change in the nervous system, which is concentrated about 

 the head and thorax. In the simplest vermiform tribes, on 

 the other hand, we lose all trace of separate ganglia, the nervous cord 

 passing without evident enlargement from one extremity to the other. 

 Whatever may be the degree of multiplication of the ganglia of the 

 trunk, they seem but repetitions of one another, the functions of each 

 segment being the same with those of the rest. The cephalic ganglia, 

 however, are always larger and more important. They are connected 

 with the organs of special sense, and they evidently possess a power of 

 directing and controlling the movements of the entire body, while the 

 power of each ganglion of the trunk is confined to its own segment. 



" The longitudinal ganglionic cord of the articulata occupies a position 

 which seems at first sight altogether different from that of the nervous 

 system of vertebrated animals, being found in the neighborhood of the 

 ventral or inferior surface of their bodies, instead of lying just beneath 

 their dorsal or upper surface. From the history of their development, 

 however, and from some other considerations, it has been suggested that 

 the whole body of these animals may be considered as in an inverted po- 



sphinx Hgustri. 



