RESPIRATORY ORGANS. 



145 



remnant of a gill the pseudobranch of the spii-acle the vessels of 

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



In the Teleosteans (fig. 593 6) and the Ganoids the lancet-shaped 

 lamella? 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- 

 culum ; this in many Ganoids and 

 Ohimsera functions as a gill, but in 

 the Teleosteans has lost its respiratory 

 function, and is then known as the 

 pseudobranch of the operculum 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 Rhinocryptis 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 FlG . 595 ._ Diafframofthe 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 Gijmnarchus is also a lung). 



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

 in A mphioxus and the LeptocephaMdce ; it circulates in a closed 



VOL. II. 10 



of aTeleostean. I', ventricle ; Ba, 

 bulbus arteriosus with the arterial 

 arches which carry the blood to 

 the gills ; Ab, arterial arches ; Ao, 

 aorta descenclens into which the 

 epibranchial arteries passing out 

 from the gills unite ; N, kidneys ; 

 D, intestine; Lk, portal circulation. 



