MORPHOLOGY OF HIGHER TYPES 123 



corrugations form box-like spaces. These corrugations increase the 

 respiratory surface. In higher vertebrates the lung is entirely subdivided 

 into minute air spaces which are in direct connection with one another 

 through large branching tubes, the bronchi. In this way there is an 

 enormous increase of the inner surface of the lung. The bronchi unite 

 in a single large tube, the trachea, which is present in the higher vertebrates, 

 but absent in some of the lower forms, as the frog. The trachea opens 

 into the mouth through a slit-like glottis. 



Accessory Respiratory Organs. — Since in all vertebrates the mouth 

 serves as a chamber for passage of water or air to gills or lungs, respec- 

 tively, it may be considered a portion of the respiratory apparatus. 

 For the same reason the olfactory passages of forms possessing lungs 

 should also be included as a part of the respiratory apparatus. Located 

 in the trachea or between the lungs and glottis is an organ of sound-pro- 

 duction, the larynx. This is a box-like structure in which are stretched 

 two membranes, the vocal cords. 



Relation to Circulation. — Since the essential part of the respiratory 

 apparatus of animals with gills or lungs is the system of blood vessels 

 with which it is richly supplied, and since the exchange of gases takes 

 place directly through the walls of the blood vessels, the respiratory 

 system is really subsidiary to the circulatory system. 



Circulatory System. — The circulatory system is not present below the 

 annelid and nemertean worms and mollusks. In the flatworms, for ex- 

 ample, there are irregular spaces in the parenchyma, the tissue of the in- 

 terior, filled with a liquid which sometimes contains granules and globules. 

 This fluid shifts back and forth with movements of the body, but there is 

 no true circulation and no true circulatory system. The essential fea- 

 ture of a circulatory system is a set of tubes which branch extensively 

 so as to bring all parts of the body in contact with the circulating medium, 

 blood or lymph: There is usually a contractile organ, the heart, or paired 

 organs which by their rhythmic contractions serve to maintain a flow of 

 the blood or lymph. The main distributing vessels are called arteries, 

 the main collecting vessels veins, and the minute tubes leading from arter- 

 ies to veins capillaries. In its course through the body the blood comes 

 into intimate contact with the walls of the digestive organs where it 

 secures dissolved food. It also comes in contact with the membranes of 

 the gills or lungs where it takes up a supply of oxygen. The blood then 

 carries this nutriment and oxj'gen out to all the tissues of the body and from 

 them brings back to the lungs and the excretory organs carbon dioxide, 

 urea, and other waste materials. In animals which have air tubes 

 (tracheae) the blood is a carrier primarily of nutriment and waste 

 materials. 



Chambers of the Heart and Course of Circulation. — The hearts of 

 various vertebrates have two, three, or four chambers, and the course of 



