NERVOUS SYSTEM OF ARTICULATED ANIMALS. 



355 



Fig. 184. NERVOUS SYSTEM OF 

 AN INSECT. 



line) in each segment; hence in the ANNELIDA and MYRIA- 

 PODA, the ganglia are very numerous ; but they are pro- 

 portionably small. In INSECTS (fig. 184), the number of 

 segments, and consequently of ganglia, never exceeds thirteen ; 

 and the ganglia are larger. What- 

 ever be the number of the ganglia, 

 they are usually but repetitions of 

 one another, the functions of each 

 segment being the same with those 

 of the rest. The nerves proceeding 

 from them are chiefly distributed to 

 the muscles of the legs ; or, where 

 legs do not exist (as in the Leech), 

 to the muscles that give motion to 

 the body. This is the case in the 

 larva of the Insect, as in the Cen- 

 tipede or Nereis ; but in the per- 

 fect Insect the case is different ; for 

 the apparatus of locomotion is con- 

 fined to the thorax ( 97), and 

 the segments of the abdomen have 



no members. We accordingly find that the ganglia of the 

 thorax, from which the legs and wings are supplied, are very 

 much increased in size, and are sometimes concentrated into 

 one mass ; whilst those of the abdomen are very small, one or 

 two of them occasionally disappearing altogether. 



441. A good example of this curious change in the nervous 

 system of Insects, is seen in the Sphinx ligustri, or Privet 

 Hawk-Moth, as shown in the succeeding diagrams. In 

 fig. 185 the nervous system of the Caterpillar is represented; 

 this consists of a pair of cephalic ganglia (1); from which 

 proceeds, on each side, a cord of communication to the first 

 ganglion of the trunk (2), and thence to the other ganglia (3 

 13). No difference is seen in these ganglia, except that 

 the last two are more closely connected than the rest. The 

 cephalic ganglia, with their cords of communication, form a 

 ring, through which the oesophagus passes ; they are situated 

 above it; but the whole chain of ganglia of the trunk is 

 situated beneath the alimentary canal. In fig. 186 is shown 

 the Nervous System of the perfect Insect; in which it is 

 seen that the whole is considerably abbreviated (the body of 



A A2 



