nt] MOUTH-PARTS AND SENSE-ORGANS 45 



convey sensations from the sense-organs, and move- 

 ments to the muscles. In the vertebrate, as the 

 reader doubtless knows, there is a brain, a nervous 

 cord running from it down the backbone, and a 

 number of nerves issuing, from the spinal cord and 

 from the brain, in various directions. Here the main 

 nervous system runs down the back of the animal. 

 In a flea, or other insect, the nervous system consists 

 of a chain of ganglia connected by a nervous cord. 

 A ganglion is a nerve centre and, in a sense, each 

 is a brain which may be likened to the one brain of 

 the vertebrate. We have in the cord of ganglia 

 a series of brains, as it were, running from the head 

 down to the extremity of the abdomen. Each ganglion 

 is a mass of nerve cells, from each of which a fibre 

 passes off to unite with the other fibres and make 

 a nerve. The first ganglion in a flea is placed in the 

 upper part of the head above the gullet. It may be 

 called the brain since it receives the nerves of the 

 antennae and eyes. In the ancestral insect we may 

 suppose that there was a pair of ganglia in each 

 segment. Since the head of the flea consists of 

 several fused segments, we may fairly draw the 

 conclusion that the brain is the result of the fusion 

 of several pairs of ganglia. 



The brain of the insect occupies the same position 

 in the body as the brain of the vertebrate ; but the 

 rest of the nervous system lies on the floor of the 



