The lungs of Serpents and Lizards are formed on a different plan. In both those groups of reptiles the dense cellular tissue is limited to the part of the lung which is nearest to the throat. This network of blood vessels and air cells extends about the principal bronchial tube much as in other animals, but as it extends backward the blood vessels become few until the _tubular_ lung appears in its hinder part, as it extends down the body, almost as simple in structure as the air bladder of a fish. Among Serpents only one of these tubular lungs is commonly present, and the structure has a less efficient appearance as a breathing organ than the single lung of the fish _Ceratodus_ (Fig. 1). The Chameleons are a group of lizards which differ in many ways from most of their nearest kindred, and the lungs, while conforming in general plan to the lizard type in being dense at the throat, and a tubular bladder in the body, give off on both sides a number of short lateral branches like the fingers of a glove (Fig. 18, p. 51). Thus the breathing organs of reptiles present two or three distinct types which have caused Serpents and Lizards to be associated in one group by most naturalists who have studied their anatomy; while Crocodiles and Chelonians represent a type of lung which is quite different, and in those groups has much in common. These characters of the breathing organs contribute to separate the cold-blooded armoured reptiles from the warm-blooded birds clothed with feathers, as well as from the warm-blooded mammals which suckle their young; for both these higher groups have denser and more elastic spongy lung tissue. It will be seen hereafter that many birds in the most active development of their breathing organs substantially revert to the condition of the Serpent or Chameleon in a somewhat modified way. Because, instead of having one great bronchial tube expanded to form a vast reservoir of air which can be discharged from the lung in which the reptile has accumulated it, the bird has the lateral branches of the bronchial tubes prolonged so as to pierce the walls of the lung, when its covering membrane expands to form many air cells, which fill much of the cavity of the bird's body (see Fig. 16). Thus the bird appears to combine the characters of such a lung as that of a Crocodile, with a condition which has some analogy with the lung of a Chameleon. It is this link of structure of the breathing organs between reptiles and birds that constitutes one of the chief interests of flying reptiles, for they prove to have possessed air cells prolonged from the lungs, which extended into the bones. CHAPTER III A REPTILE IS KNOWN BY ITS BONES