RESPIRATORY ORGANS. 



145 



remnant of a gill the pseudobranck of the spiracle the vessels of 

 which belong to the arterial circulation and form a rete mirabile. 



In the Teleosteans (fig. 593 b) and the Ganoids the lancet-shaped 

 lamellae are arranged in double rows 

 on the four visceral arches which func- 

 tion as branchial arches, and they form 

 four comb-shaped gills on either side. 

 These gills lie in a spacious branchial 

 cavity covered by the branchial oper- 

 culum and the branchial membrane. 

 There is, however, an accessory gill on 

 the inner side of the branchial oper- 

 culurn; this in many Ganoids and 

 Chimsera functions as a gill, but in 

 the Teleosteans has lost its respiratory 

 function, and is then known as the 

 pseudobranch of the opereulum or of 

 the hyoid arch. 



External gills projecting from the 

 slits of the branchial pouches are found 

 only in the embryos of the Plagiostomes. 

 Rudiments of external gills are found 

 in Rhinocr^ptis annectens. 



Finally the secondary cavities, which 

 are sometimes found annexed to the 

 branchial cavity and increase the re- 

 spiratory surfaces by the development 

 of a capillary network, must be regarded 

 as accessory organs of respiration. They 

 consist either of labyrinthine cavities 

 in the superior pharyngeal bones (fig. 

 594) or of saccular appendages of the , 



FIG. 59o. Diagram of the circulation 



branchial cavity (Saccobranchus, Am- 

 phipnous). True lungs derived from 

 the swimming bladder, with internal 

 cellular spaces, a short air-tube and 

 glottis-like opening into the pharynx, 

 are only found in the Dipnoi (according 

 to Hyrtl the swimming bladder of Gymnarchus is also a lung). 



Vascular system. The blood is generally red; it is white only 

 in Amphioxus and the LeptocephaUdm ; it . circulates in a closed 



VOL. ii. 10 



of aTeleostean. V, ventricle ; Ba, 

 bulbus arteriosus with the arterial 

 arches which carry the blood to 

 the gills ; Ab, arterial arches ; Ao, 

 aorta descendens into which the 

 epibranchial arteries passing out 

 from the gills unite ; N, kidneys ; 

 Z>, intestine; Lk, portal circulation. 



