318 THE ZOOLOGIST. 
alone (although, indeed, there are others in plenty) that we 
should find in it a very primitive condition of the respiratory 
apparatus. Scattered over the whole of the body there are 
minute orifices ; these each lead into a pit which widens out at 
its base; from this base there are given off a number of delicate 
tubes which make their way into the organs of the body ; it is by 
their means that the necessary oxygen is conveyed to the 
tissues and cells of the organism. 
As we pass from this low and primitive form to the more 
highly organised Insects we find, as elsewhere in the Animal 
Kingdom, and as with other organs, that diffuseness and 
simplicity give place to concentration and complexity. The 
number of orifices becomes greatly reduced, so that at last there 
are but two or three pairs; the tubes themselves become 
strengthened and kept open by a chitinous fibre which extends 
spirally along them, and the supply of air to the whole body is 
effected by the branching of the trunks and their union with 
one another. . 
If I were to try to sum up what I have attempted to tell you 
to-day I should put the result thus :— 
Respiration, or the exchange of fresh oxygen for the carbonic 
acid which is constantly formed as one of the waste products of 
the activity of all living cells, is to be observed in every living 
organism ; like every other function, it is performed in a vague 
manner-—or without the aid of special organs—by the lowest. 
In the higher there are special parts of the body allotted for this 
function, and these may be outgrowths or ingrowths. These 
organs are, in a large number of cases, aided by certain 
chemical compounds which have the office of oxygen-carriers ; 
that is to say, they are capable of storing up oxygen and 
of giving it up again, and they are able to repeat this alternation 
of conditions repeatedly, and throughout life. Indeed, without 
them the life of the higher organisms would be impossible. 
