416 Transactions of the Royal Society of South Africa, 
continual working of a special oar-like organ. In fishes, also, exposure of 
the blood is carried on by means of gills which are situated in the head- 
region and enclosed in a gill chamber. This chamber is in connection 
with the cavity of the mouth. Water is taken in by the mouth, the 
mouth closed, and the water forced through the gill chamber, the ali- 
mentary canal, of course, being kept closed during the process. This 
opening and closing of the mouth has become quite mechanical in fishes. 
Eespiration being taken to mean the interchange of gases taking place in 
each individual living cell, it cannot be correct to consider the gills of 
fishes, &c., as respiratory organs. The gills furnish a medium for the 
interchange of gases between the blood and the water, and this process 
may therefore be termed breathing as distinct from respiration. 
In insects the blood does not go to any specialised breathing organ, 
but instead there is found ramifying throughout the body a system of fine 
tubes carrying air and connected with the exterior. These tubes, so to 
speak, themselves seek out the blood and the blood tissues in every part of 
the insect's body, penetrating to the tips of the antennae and of the legs 
and entering the most delicate tissues. The want of an arterial system 
is compensated for as well as conditioned by the extraordinary profuse 
branching of these tubes or trachea. Most aquatic larvae of insects breathe 
by means of tracheal gills or branches, which are either filamentous or 
leaf-like appendages containing trachea. In some insects such as the 
dragon-fly and grasshopper there is a rhythmical rise and fall of the 
upper and lower walls of the abdomen, during which the air enters and 
passes out of the air openings or spiracles on each side of the body. 
In Frogs and Toads, although breathing in the tadpole stage is by 
means of gills, the land habit of the adult requires a different means, as 
the external medium is now air instead of water. For this purpose two 
large distensible air sacs are found inside the front part of the body and 
connected directly to the cavity of the mouth. These sacs are emptied 
and filled with air alternately by mechanical movements connected with 
the floor of the mouth and nostrils. These sacs are called lungs, and their 
internal surface is somewhat increased by ingrowths from the walls. It is 
to the walls that deoxygenated blood is sent from the heart and distributed 
by fine capillaries, and it is there that the interchange of gases in the 
blood takes place. The oxygenated blood from the lungs then returns to 
the heart to be sent to all portions of the body. The heart is now a highly 
difl^erentiated muscular organ. In these animals a certain amount of 
breathing is also carried on in the skin where an interchange of gases 
between air and blood can take place. This is said to be the only means 
of breathing during hibernation in winter. In birds the lungs are solid, 
spongy organs, only slightly distensible, in which air tubes ramify, the tiny 
branches ending in small blind dilatations known as alveoli. It is in these 
