242 BOWERBANK ON 



the place of that absorbed by the valves of the great dorsal 

 vessel; c, Fig. 1, shews the course of these vessels, and d, 

 Fig. 1, the points at which they communicate with the abdomi- 

 nal cavity. These vessels terminate at e, Fig. 1, by discharg- 

 ing their contents into the lower end of the great dorsal vessel. 



The circulation is also strikingly and beautifully exhibited 

 in the tail. Here the ascending and descending vessels, like 

 vein and artery, accompany each other; and, at the same 

 instant that the blood is seen to pass up the one, with the usual 

 pulsatory motion, it descends in the other in a similar manner. 

 This is the more apparent, as the sides of the vessels are well 

 defined, and each perfectly distinct from the other. 



Although the blood passes with the same pulsatory motion 

 through these minute vessels as it does in other parts of the 

 body, yet no pulsation of either the ascending or descending 

 vessels themselves can be detected. The motion, therefore, 

 seems to be entirely dependent on the action of the great 

 dorsal vessel, which evidently performs in the insect the same 

 functions that the heart does in vertebrated animals. 



Supplying and returning vessels may also be seen in the 

 legs, although they cannot be so clearly defined as in the tail ; 

 and in the antennae they pass up on the one side of the first 

 joint, and, turning round at the extremity, they again descend 

 into the head. 



Upon fixing the insect so as to obtain a side-view, the great 

 dorsal vessel presents a very interesting appearance. It is seen 

 continually and regularly oscillating backwards and forwards, 

 upwards and downwards, and at the same time the main current 

 of the blood in the great abdominal cavity winds its way in all 

 directions towards the hinder extremity of the insect. Scarcely 

 any larvae exhibit the circulation of the blood in so beautiful a 

 manner as the one described, although there are few in which 

 it is not more or less to be seen, as I have been able to detect 

 the great dorsal vessel in almost all I have examined, In 

 one, figured in the work before quoted of Dr. Goring and 

 Mr. Pritchard, and said to be the larvae of a Culex, no particles 

 are visible in the blood ; but the great dorsal vessel, its valves, 

 and their singular appendages, are distinctly and beautifully 

 apparent. 



Next to the larvae of the Ephemera marginata, the larvae of 

 Agrion affords the best view of the blood and its circulation. 



