therefore, must have, in addition toits mechanisms for bringing food and oxygen to the cells, a means for the re- moral of wastes. The supplying of oxygen and the removing of carbon dioxide and some of the excess water are accomplished by respiration. Respiration is primarily the exchange of gases between the cells of the body and the outside air. If an animal is suflîciently small and soft-skinned, the gas ex- change can be made directly by diffusion through the skin. Larger animais, however, must have a device for conveying air into the body where the tissues wil| have closer access toit. It will be evident, then, that there is not neces- sarily only one way of accomplishing the purposes of respiration. Vertebrate animais inhale air into a sac or pair of sacs, called the lungs, through the very thin walls of which the oxygen and carbon dioxide can go into and out of the blood respectively. The blood contains a special oxygen carrier in the red matter, hemog|obin, of its red corpuscles, by means of which the oxygen taken in from the air is transported to the tissues. The carbon dioxide is carried from the tissues partly by the hemoglobin, and partly dissolved in the blood liquid. Insects have no lungs, nor have they hemoglobin in their blood, which, as we have seen, is merely the liquid that fills the spaces of the body cavity between the organs, lnsects have adopted and perfected a method of getting air dis- tributed through their bodies quite different from that of the vertebrates. They have a system of air tubes, called tracheae (Fig. 7o), opening from the exterior by small breathing pores, or spiracles (Sp), along the sides of the body, and branching minutely within the body to all parts of the tissues. By this means the air is conveyed directly to the parts where respiration takes place. There are usually in insects ten pairs of spiracles, two on the sides of the thorax, and eight on the abdomen. The spiracles communicate with a pair of large tracheal trunks lying
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