RESPIRATORY SYSTEM 761 



after enlarging, slides back over those lying more caudad so that at least 

 the external branchial grooves lie in a pocket called the cervical sinus. 

 This sinus is later closed by a process from the hyoid arch extending 

 over it quite similarly to the development as shown in Anura. Inter- 

 nally the entodermal branchial pouches, with the exception of the first, 

 disappear, but the first persists as the Eustachian tube and the greater 

 part of the middle ear. 



THE SWIM BLADDER 



The swim bladder arises as a diverticulum of the alimentary canal 

 remaining in contact with that canal by a pneumatic duct in the ganoids 

 and one group of teleosts (Physostomi), (Fig. 441). This duct, although 



Fig. 441. 



Swim-bladders of those fresh-water fish whose air-bladders have 

 a duct (physostomous) . A, Pickerel; B, Carp; C, Eel. b, swim- 

 bladder; d, duct; g, red gland; oe, oesophagus. (From Kingbley 

 after Tracy.) 



usually emptying into the oesophagus, may connect with the stomach. 

 However, in most teleosts the duct disappears entirely at an early date. 

 The swim bladder lies dorsal to the digestive duct outside of the peri- 

 toneum, although below the vertebrae and excretory organs. It may be 

 of almost any dimensions, sometimes extending the entire length of the 

 body. In some forms of teleosts, which remain almost constantly at the 

 bottom, it is absent entirely. The swim bladder, although usually un- 

 paired, is paired in most ganoids and may even form three divisions of 

 connecting sacs. There may be diverticula of any and all kinds. The 

 internal part of it may be smooth and simple or it may be subdivided by 

 various septa, or it may even be alveolar resembling the lungs of higher 

 vertebrates. There may be striated muscle fibers in the walls, and in 

 some Siluroids and Cyprinoids they are even somewhat calcified on ac- 

 count of some of the vertebral processes being included. 



The blood supply of the swim bladder is arterial, and comes from 

 either the aorta or the coeliac axis ; sometimes different portions receive 

 blood from both these vessels. The arteries break up in the walls to 

 become networks of minute vessels known as retia mirabilia. These often 

 form "red spots" on the inner surface. From the retia the blood passes 

 to the postcardinal, hepatic or vertebral body-veins, in the ganoids and 

 phystomous species, especially those with a wide pneumatic duct. 



The swim bladder contains a greater quantity of O 2 than is found 

 in solution in the water in which the fish lives. It is therefore probably 

 a storage organ for O 2 for use when the fish dives to lake bottoms in 



