144 BIOLOGY: GENERAL AND MEDICAL 



size, number, distribution, structure, and arrangement 

 of the branchiae undergo great modification, but they 

 continue to be the only means of effecting aeration of the 

 blood so long as aquatic life continued. Modified 

 branchiae, indeed, persist among terrestrial mollusks. 



In general arrangement the branchiae consist of more 

 or less well-protected, simple or complex surfaces upon 

 which the blood of the animal is brought to the surface 

 of the body, and in intimate contact with the surround- 

 ing water in order that the exchange of gases may be 

 effected. 



So soon as terrestrial life is adopted lungs are de- 

 veloped, and the atmosphere rich in oxygen is taken into 

 the body and there aerates the blood. Lungs at first ap- 

 pear as relatively simple sacs into which air is drawn, 

 and in the walls of which innumerable capillaries ramify. 

 Soon, however, the structure becomes more and more 

 divided into minute sacs or alveoli, in the walls of 

 which the capillaries ramify so that the amount of aerat- 

 ing surface is enormously increased and the gaseous 

 exchange made correspondingly easy. 



With the development of the lungs special means 

 must be provided for creating the necessary vacuum 

 by which the air is to be drawn in. In the lower verte- 

 brates (reptiles) among whom the breathing is slow 

 and not very regular this is accomplished by the com- 

 bined movements of many of the body muscles, but in 

 the higher vertebrates the body cavity is divided by a 

 transverse muscular partition, known as the diaphragm, 

 whose contractions and relaxations are the chief source 

 of the respiratory movements. 



The Excretory System. Vital activity, being a chemi- 

 cal process effected through the oxidation of protoplasm, 

 is inevitably attended with the formation of combus- 

 tion products. Of these the organism can make no 

 further use, partly because their molecular structure 

 is more stable than that of the protoplasm itself and 

 partly because the energy required to resynthesize them 



