Respiratory System. 175 



tine. This indicates the origin of the bladder to be, as Huxley slates it, 

 a mere " diverticulum " of the intestine. In the Herring, Carp, &c. , 

 as well as the Ganoids, this tube is useful and persistent through life, 

 while in the Cod and Perch, the tube becomes obliterated and the blad- 

 der remains a closed sac. There are several indications of retrograde 

 development in certain particulars in some of the Teleostean fishes and 

 this appears to be one. 



The Dipneusta, including the Lepidosiren and Ceratodus, constitute 

 the connecting link between the Ganoid fishes and the Amphibians. 

 Like the Ganoids below them they are in one sense Amphibians. In 

 them the swim bladder is a double organ and cellular and connects with 

 the throat by a tube -for breathing. It is the full equivalent of the 

 lung. Yet the animals retain the gills, and both organs are used. In 

 some of the Amphibians the gills only are used in the youthful stage 

 and the lungs only in the adult stage. In all the vertebrates above the 

 Amphibians the gills are developed in the embryo stage, and the blood 

 vessels run to them as if to receive their respiration there as of old, but 

 the lungs are also developed in the embryo, and before birth the gill 

 arches are turned to other use and after birth the lungs alone are de- 

 pended on. It may be observed that in connection with the complete 

 development of the swim bladder into lungs in the Lepidosiren, the 

 nasal cavity is opened back into the throat so that air can get into the 

 lung without the necessity of the mouth being opened, as the case is 

 with other fishes. 



It is to habit and use therefore that we must attribute the building up 

 of the lungs or other breathing apparatus, as well as the specialization 

 of the various parts of the system, limbs, &c. , as pointed out above. 



First his environment driving the aquatic animal to greater activity 

 whereby his system undergoes greater exhaustion, he automatically 

 offers a greater amount of the carbon of his wasted tissues to the chem- 

 ism of oxygen. The animal learns by experience that the sensation of 

 uneasiness caused by suffocation is relieved at the surface of the water. 

 Accordingly he goes there and exposes himself to the action of the air, 

 some of which reaches his gills and some his stomach. The oftener 

 this is done the more active will the fish become, and conversely the 

 more active he is the oftener will he repeat the habit. The oftener the 

 habit is repeated the more specialized will the part become which is 

 exposed to the chemical interchange between the 'two elements. The 

 intimate and ultimate nature of what is called habit will be discussed 

 further on. In the meantime we are learning that use and habit are the 

 immediate potent factors in building up organisms and effecting the spe- 

 cialization of organs and parts, while disuse and inertion are negative fac- 

 tors permitting the subversion and gradual atrophy of parts once built up. 



