OF THE TRACHEA. 191 



more or less to be seen, as I have been able to detect the 

 great dorsal vessel in almost all I have examined. 



" Next to the larva of the Ephemera marginata, that of 

 Agrion affords the best view of the blood and its circula- 

 tion. In all the species of these larva I have yet exam- 

 ined, I have found it as nearly similar as possible in 

 appearance to that which we observe in the Ephemera, 

 and, in some instances, it has afforded even more satis- 

 factory results. The head of this larva is much more 

 transparent than that of the larva of the Ephemera, we 

 therefore have a better view of the circulation of the blood 

 in the head of this insect than 'can possibly be obtained 

 in the other. In this object the blood is seen rushing like 

 a beautiful intermittent fountain towards the mouth, and 

 dividing, right and left, into two jets, a portion of each of 

 which flows within a given boundary past the back of the 

 eye, whilst the remainder winds its way through other 

 channels, deep in the side of the head, and returns again 

 into the body. The antennas of this insect also afford 

 another beautiful instance of the circulation being carried 

 forward within well-defined vessels. They are each com- 

 posed of six joints, up four of which the blood is seen to 

 take its course, and after turning round the extremity of 

 the fourth joint, it returns by a distinct canal into the head. 



" In the leg, likewise, the circulating fluid and its canals 

 are clearly and distinctly to be traced, even to the very 

 extremity of the tarsus, where, as in the antennae, the par- 

 ticles of the blood are seen to descend on the one side of b* 

 the leg, and, turning the extreme point, to return up the 

 contrary side to the one by which they came down." 



The Tracheae. In vertebrated animals, the lungs are 

 situated in the anterior part of the trunk : the whole of the 

 air that is appropriated to the uses of the body must be 

 inspired by the trachea, which ascends from the lungs to 



