400 MANUAL OF ZOOLOGY SECT. 



front of, the urinogenital aperture, there being no cloaca 

 such as occurs in Elasmobranchs. 



The gills are usually comb-like, consisting of rows of 

 slender branchial filaments. The interbranchial septa are 

 reduced as compared with those of the Elasmobranchs, the 

 branchial filaments projecting freely beyond them. As a 

 rule the gills are developed on the first four branchial 

 arches. 



A characteristic structure of the Teleostomi is the air- 

 bladder or swimming bladder, which, however, is not present 

 in all. It is an elongated sac with elastic walls situated in 

 the body-cavity immediately below the spinal column. In 

 some cases (Ganoids and some Teleostei) it communicates 

 with the pharynx by a duct, the pneumatic duct ; in the rest it 

 is a closed sac. It is sometimes divided into compartments, 

 or produced into lateral offshoots. In some of the Ganoids 

 its wall is sacculated, assuming an appearance not unlike 

 that of the lung of one of the higher air-breathing Vertebrates. 

 The air-bladder seems able to act as a sort of accessory 

 organ of respiration. Its normal function, however, appears 

 to be hydrostatic, i.e., it serves to keep the fish of the 

 same specific gravity as the water ; variations in pressure as 

 the fish ascends or descends are regulated by absorption or 

 secretion of gas. 



In the Ganoids the heart has a structure very similar to 

 that of the Dog-fish, consisting of a sinus venosus, auricle, 

 ventricle and conus arteriosus the last being rhythmically 

 contractile like the other chambers, and containing rows of 

 valves. In Teleostei there is no such conus arteriosus ; but 

 there is always a large bulb-like dilation of the base of the 

 ventral aorta, the bulbus aortce. The optic nerves of the 

 Ganoids agree with those of Elasmobranchs and of Verte- 

 brates in general in forming a chiasma, whereas in the 



