2 BOARD OF AGRICULTURE. 
ternal organs. The nervous system consists of 
a double nerve-centre or ganglion in the upper 
part of the head, connected with another below 
by nerves that pass around the gullet on each 
side, so as to enclose it as in a ring, and this 
lower double ganglion is connected by a double 
-nervous cord with a series of similar ganglions 
or nerve-masses, which are arranged along the 
lower side of the body, below the intestine, each 
ganglion sending off small nerves to the adja- 
cent organs (figure 2). The rings of the body 
are thickened in certain parts, and in the head 
and middle region of the body are more or less 
consolidated. They thus form a somewhat firm 
external skeleton, composed of 
a peculiar substance, called 
chitin. When the rings are 
movable this outer covering, 
where it connects the rings to- 
gether, is thinner and flexible, 
and often folded inward so as 
to form peculiar joints. The 
legs, jaws, and other external 
organs, are merely hollow out- 
foldings of the outer covering, 
and contain, like the body, the 
muscles that move the various 
parts, while their cavities com- 
municate freely with that of 
the body, and thus receive their 
supply of blood, nerves, etc. 
Each joint of the legs and other 
appendages is also formed by 
a thinner portion of the outer 
covering, which is folded in 
‘upon itself in various ways. 
The wings, when present, are 
also outfoldings of the Integy joosote 
For description of Figures see next page. 
Fig, 2. 
