LAEVAL GILLS OP THE ODONATA. 417 



"We hope to show in a subsequent paper, however, that the 

 existence of such a mechanism is quite possible though it can- 

 not be very efficient. Being thus led to enquire whether there 

 are no other organs to help in the excretory activity of the gill, 

 we have discovered, and intend soon to describe more fully, 

 two very remarkable and quite enigmatical organs in a part of 

 the digestive tract, which we propose to term the prcerectal 

 vesicle. 



The organs in question consist of two discs of very peculiar 

 epithelial cells which depend from the wall of this vesicle. They 

 appear to be non-glandular, and their function is quite unknown. 

 "We have on several occasions found the prasrectal vesicle filled up 

 and considerably swollen by a gaseous contents, and we incline 

 to the belief that the function of the " discs," as well as of other 

 productions in the basal part of the gill which are covered with 

 the same epithelium, may be the excretion of carbonic acid, but 

 we put this forward merely as a hypothesis, pending experi- 

 mental research on the subject. 



In all non-tracheal gills, as well as in Arachnidan lungs, the 

 blood plays a very important part indeed in the process of respi- 

 ration — that of collecting and carrying away the oxygen to all 

 parts of the body. The osmotic process, on current theory, is 

 supposed to take place between the outer atmosphere or water 

 and the blood itself, through the cellular and cuticular wall. 

 Now, if there is a blood-space in the gill of Libellula, it may be 

 thought likely that the same process must take place there, just 

 as in the gills of Limulus and Isopods, or in the lungs of spiders, 

 because the same causes must produce similar effects under similar 

 circumstances, and a certain foundation cannot be refused this 

 hypothesis. We may remark, however, that the circumstances 

 are not exactly the same in tracheal and non-tracheal organs. 

 The presence of numerous tracheal loops in the protoplasmic 

 coating of the gill may alter considerably the conditions of the 

 process, and it could alter them even if it were a mere physical 

 one. But, knowing that this respiratory process is neither 

 so clear nor so simple as it is often said to be, we cannot refrain 

 from thinking that the functional protoplasm casts the greatest 

 part of the absorbed oxygen, if not the whole of it, into the 

 tracheal tubes that must carry it to every organ in the body, 

 and that the blood would seem to play a very unimportant part, 

 if any, in the absorption of that gas. On the other baud, it must 



