310 SYSTEMATIC ZOOLOGY. 



above, becoming flatter by contraction and thus enlarging 

 the cavity (thoracic cavity) which lies in front of it. 



In the ganoids and bony fishes exists a structure, the 

 swim-bladder or air-bladder, which is usually thought to 

 represent the lungs. In the lower teleosts (Physostomi) 

 it is connected with the alimentary canal by a duct open- 

 ing on the dorsal wall of the pharynx, but in others (Physo- 

 clisti) this duct closes long before the adult condition is 

 reached. In the lung-fishes, on the other hand, the 

 structure is double and its duct ventral. 



Connected with the respiratory system are two glands of 

 problematical function. One of these, the thyroid, is 

 formed from the floor of the pharynx. The other (the 

 thymus) arises from the gill-pouches, and in the higher 

 vertebrates disappears in adult life. In the calf it forms 

 the 'neck sweetbread.' Both these glands are without 

 ducts, and the part they play is obscure, but since when 

 the thyroid is diseased it produces serious illness, it is 

 apparent that it is very important in the economy. 



In the circulatory system three parts may be recognized: 

 (1) a central propelling organ, the heart; (2) arteries, 

 carrying the blood away from the heart; and (3) veins 

 bringing it back. Between arteries and veins are inter- 

 posed minute tubes, the capillaries. 



The heart is a muscular organ, enclosed in a special sac 

 of the body-cavity, the pericardium. In the heart can 

 always be distinguished a receptive portion (auricle), which 

 receives the blood as it comes from the veins, and passes 

 it on to the true propelling organ, the ventricle. This 

 latter has strong muscular walls, and when it contracts, 

 the blood, prevented by a valve from returning to the 

 auricle, is forced out through the artery (ventral aorta) 

 connected with the ventricle. 



