NERVOUS SYSTEM OF ARTICULATED ANIMALS. 355 
line) in each segment; hence in the Annelida and Myria¬ 
pod a, 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 gangliaare 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 
eephalic 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 a 2 
Fig. 184 .—Nervous System of 
an Insect. 
