xii PHYLUM CHORDATA 403 



comb-like, consisting of rows of slender branchial filaments. 

 The inter-branchial septa are reduced as compared with 

 those of the Elasmobranchs, the branchial filaments project- 

 ing freely beyond them. As a rule, the gills are developed 

 in 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 gullet 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 verte- 

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

 accessory organ of respiration. Its normal function, how- 

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

 of the same specific gravity as the water ; variations in press- 

 ure, as the fish ascends or descends, are regulated by ab- 

 sorption or secretion of gas. 



In the Ganoids the heart has a structure very similar to 

 that of the dogfish, 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 aorta. The optic nerves of the 

 Ganoids agree with those of Elasmobranchs and of Verte- 

 brates in general in forming a chiasma, whereas in the 

 Teleostei they simply cross one another or decussate. 



Most Teleostomi are oviparous, the eggs being impreg- 

 nated after they are laid. Many instances of parental care 



