84 
ON THE BEEATHING APPABATUS 
easily observed, the sand being spirted up at every movement of 
the animal. 
It is within this sac that the breathing organs of MscJina, 
Libelkila, and Calopteryx are situated, but to study the disposal 
of them it is necessary that the larva should be dissected. To 
do this it is best to fix the animal on its back, and open the 
abdomen all round the lateral seam with curved scissors, taking 
great care not to injure any of the internal organs in so doing ; 
then turn back the skin and cut it off close to the thorax ; 
having done this, the disposition of the intestines, trachea, and 
respiratory sac may be observed with the greatest facility. 
First, close to the thorax is the stomach, leading into the 
small intestines, to which are attached the peculiar filamentous 
bodies which are generally called Biliary canals further on 
the large intestine, and then the sphincter ani; this opens into 
the breathing sac at its anterior extremity, and all foecal matter 
on passing into this sac is instantly discharged at the posterior 
valve by a rapid contraction of the abdomen, similar to that by 
wFich the animal propels itself through the water. 
Thus it will be noticed that this sac is not identical with the 
large intestine, as stated by Dr. Duncan and others, but is 
supplementary ; the real sphincter ani being situated at its 
anterior extremity. 
In the case of Agrion, however, this sac is altogether 
wanting, so far as I have been able to observe, and the quick 
movement wFich it makes when escaping an enemy, is managed 
by means of three broad leaf-like appendages situated on the end 
of the tail ; these are spread out, somewhat like a chestnut leaf, 
and suddenly are brought together backwards, thus acting as 
paddles in sending the insect through the water ; these append- 
ages are strongly keeled, to give them more power of resistance. 
If these plates be examined, it will be found that they are 
filled with trachea ramifying over the whole surface, and it is by 
