DISTINGUISHING CHARACTERS OF INSECTS. 21 



Taking the nervous system next, we find an important 

 difference between the vertebrate animals and insects. 

 In insects, instead of one nervous centre or brain, a 

 series of nerve-knots, called ganglions, communicating 

 with each other, and yet acting apparently with some 

 degree of independence, send off the supply of nerves 

 required by the body. Thus, while a vertebrate animal dies 

 at and below the point at which the connexion with the 

 brain is cut off, an insect may be cut into several pieces, 

 and to all appearance each may, for a considerable time, 

 show signs of vitality. Thus a headless insect may be 

 seen to walk ; and a dragonfly, accidentally divided into 

 three parts, the head, the thorax, and the abdomen, has 

 for days kept up considerable action in the separated 

 parts, the wings fluttering violently on any attempt to 

 confine them, and the abdomen wriggling when touched. 



The circulating and breathing systems are next to be 

 compared. 



Until recently (i.e., within the last fifty years) insects 

 were believed to be without any heart or circulating 

 system whatever, although a certain motion of fluids 

 had been observed before that time. Now, however, it 

 has been shown that a long muscular vessel, which is 

 in fact a sort of compound heart, or series of heart- 

 valves terminating in a large artery, runs from the end 

 of the body into the head. Here this vessel branches 

 off, and although from the extreme delicacy of its minute 

 offshoots these have been traced but a little way, there 

 seems reason to believe that a system of arteries and 

 veins exists on the same principle as in the vertebrata. 



The hearts of the vertebrata and of insects are not 

 more unlike than are the organs of respiration. In the 

 place of the lungs, two large spongy bodies, full of air- 



