Sy LOMO OGY 
GHA FER 
CLASSIFICATION 
At the outset it 1s essential to know where insects stand in 
relation to other animals. 
Arthropoda.—Comparing an insect, a. centipede and a 
crayfish with one another, they are found to have certain 
fundamental characters in common. All are bilaterally sym- 
metrical, are composed of a linear series of rings, or segments, 
bearing paired, jointed appendages, and have an external 
skeleton, consisting largely of a peculiar substance known as 
chitin. 
If the necessary dissections are made, it can be seen that 
in each of these types the alimentary canal is axial in position: 
Diagram to express the fundamental structure of an arthropod. a, antenna; al, 
alimentary canal; b, brain; d, dorsal vessel; ex, exoskeleton; J, limb; n, nerve chain; 
s, subcesophageal ganglion.—After SCHMEIL. 
above it extends the dorsal blood vessel and below lies the 
ventral ladder-like series of segmental ganglia and_ paired 
nerve cords, or commissures; between the commissures that 
connect the brain and the subcesophageal ganglion passes the 
cesophagus. These relations appear in Figs. 1 and 163. 
2 I 
