How fish breathe
The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per litre of water. This is much less than the 210 ml. of oxygen per litre of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one we found in the fish's swim bladder and in the tuna's muscles. The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. This is a very simple closed-circle circulatory system. The gills: the gills are composed of a gill arch (which gives the gill rigid support), gill filaments (always paired), and secondary lamellae, (where gas exchange takes place).
How locusts breathe
Locusts, like most insects, have what entomologists refer to as a ventilatory system. Instead of using a circulatory system and hemoglobin like you and me, most insects directly vent every cell with the outside environment and exchange oxygen directly. Along an insects' side, there are small holes called spiracles; these are the openings to the ventilatory system and there are usually two per segment: one on each side. The spiracles often have some sort of valve that allows the insect to close off its ventilatory system- this is analogous to holding your breath. I've seen some insects stay lively in a jar full of ethyl acetate (the gas I use to knock them out) for an hour or longer! Inside of the spiracles are the tracheas, which branch out into smaller...
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