INSECTS 345 



so that the supply of oxygen and the removal of carbon 

 dioxide is constant. In the oyster the system is similar, 

 except that there are no tubes, as the movement of the 

 sea water is sufHcient for this renewal and removal. 



In the air breathing mollusks, of which the snails are a 

 type, the blood vessels line a cavity in the body, and this 

 cavity is connected with the outside by an opening. This 

 cavity is an infold of the skin instead of an outfold, but its 

 action is identical with that of the gill. Into this aperture 

 the snail draws a bubble of air, and even the water snails 

 creep to the surface for this purpose, and sink to the bot- 

 tom again after filling the cavity. The blood vessels 

 which line this cavity effect an interchange of gases with 

 this air just as the gills do with the water, and after all 

 the oxygen is removed the walls of the cavity contract 

 and force out the bubble, now laden with carbon dioxide, 

 and take in a new supply of air. This is the simplest form 

 of a lung loiown. 



Insects. — In the insects we find air breathers having a 

 respiratory system unlike that of any other animal. These 

 air breathers have openings called spiracles along the sides 

 of the abdomen. By movements of the body these spir- 

 acles can be made to open and close rhythmically. They 

 are actually the mouths of a system of branching tubes or 

 tracheae which penetrate to all parts of the body, and thus 

 carry air directly to the various body tissues. These tubes 

 are strengthened internally by a spiral thread which 

 prevents their collapse as the air rushes in and out. The 

 insects are the only animals that carry air directly to -all 

 parts of the body. The movements of the abdomen 

 maintain a constant circulation of air through these tubes, 

 in and out of the spiracles. This system is entirely inde- 



