IV. VERTEBRATA: PISCES. 



567 



Fio. 595. Sections of gill 

 arches of Gadm (left) 

 and Zygcena (right), 

 slightly enlarged, a, 

 artery ; ft, gill arch ; 

 W*f bi 2 , demibranchs ; 

 h, dermal projection ; 

 r, cartilage ray; V, 

 vein ; z, tooth. 



(r&r) from the hyoid, these latter supporting the branchiostegal 

 membrane. Between the free edge of the 

 operculum and the branchiostegal mem- 

 brane and the skin of the body behind is 

 the opercular cleft (fig. 594, ops), which is 

 obviously not identical with a gill cleft, but 

 leads into an atrium into which the gill 

 clefts empty. In many elasmobranchs and 

 ganoids there is a rudimentary cleft, the 

 spiracle, between the pterygoquadrate and 

 hyomandibular, in which a rudimentary gill, 

 or pseudobranch, may occur, this often per- 

 sisting when the spiracle is closed. 



Besides gills, fishes, with .the exception 

 of elasmobranchs and some teleosts, have a 

 swim bladder which is usually regarded as 

 the homologue of the lungs. It is often 

 shaped like an hour glass, filled with air, and 

 may open into the oesophagus by a pneumatic 

 duct (Physbstomi), or this, appearing in development, may be lost 

 in the adult (Physoclisti). The air bladder serves for respiration 

 in the Dipnoi and possibly in some ganoids (Lepidosteus and 

 Amia), but is usually a hydrostatic apparatus, its enlargement or 

 compression altering the specific gravity of the fish. In fishes 

 brought up from great depths the expansion of air in the swim 

 bladder frequently forces the viscera out through the mouth. 



The heart, enclosed in the pericardium, lies immediately 

 behind, the gill region, and is protected by the shoulder girdle. 

 It always consists of auricle and ventricle (fig. 596), separated by 

 a pair of valves to prevent back-flow of the blood; it sends the 

 blood to the gills by the arterial trunk (ventral aorta), and receives 

 it from the body through a thin-walled sac, the venous sinus, in 

 which the hepatic veins and the Cuvierian ducts (formed by union 

 of jugular and cardinal veins) empty (figs. 65, 597). 



The most important differences lie in the development of conus 

 and bulbus arteriosus. These are muscular accessory organs, the 

 first arising from the heart, the other from the arterial trunk; and 

 correspondingly the conus has striped, the bulbus smooth muscle 

 fibres. The anterior end of the heart contains < semilunar ' valves, 

 which, like the auriculo-ventricular vjilves, prevent the back-flow 

 of the blood. When, by increase in the number of valves, this part 

 becomes elongate, a conus arteriosus (fig. 596, A) is formed. The 



