INTERNAL ORGANS. 85 



modified, however, to form a continuous, closely 

 coiled, spiral thread, making an elastic cylinder ; 

 this does not continue throughout the entire 

 trachea, but disappears when the finer ramifica- 

 tions are reached, leaving only the softer epider- 

 mal integuments ; these apical portions of the 

 trachea! vessels, as has been said, are found every- 

 where, piercing the fatty lobules, clasping all the 

 interior organs, tracking along every nerve and 

 muscular fibre, penetrating every tissue, and ter- 

 minating in a mesh- work of interlacing branches, 

 their surfaces everywhere bathed in the fluids 

 which fill the body cavity and flow gently from 

 head to tail by the pulsatory action of the dorsal 

 vessel. Oxygenation, then, is effected by the in- 

 terchange of gases through the delicate mem- 

 branes investing the ultimate tracheal branches, 

 and when effected the purified fluids are at hand 

 to build up the tissues of the body, and have not 

 to be forced to the needed spot by arteries. 



The foundation of the nervous system of cater- 

 pillars is a series of minute horizontal disks or gan- 

 glia lying on the floor of the body cavity along 

 the middle line, and connected with one another 

 by a pair of slender cords, lying side by side ; as 

 this series of disks and connecting cords enters 

 the head, it forms a ganglion just beneath the 

 oesophagus, beyond which the cords embrace the 

 oesophagus, and again unite above in a pair of 



