424 THE ANIMAL KINGDOM. 



Amia and chondrostean Ganoidei, but a long one, the ductus pneumaticus, 

 is found in the Teleostean sub-order Physostomi, while it is absent in the 

 remaining sub-orders, hence often grouped together as Physoklisti. The 

 lumen of the duct is, however, sometimes closed. The sac itself is double 

 in PolypteruS) Protopterus, and Lepidosiren, single in other Fish : it lies 

 between the alimentary tract below, the aorta and kidneys above : it is 

 covered by peritoneum only on its ventral surface, and is sometimes pro- 

 tected in Cyprinoidei and Siluroidei by a partial or complete bony capsule. 

 Its inner surface is produced into ridges, regular in Lepidosteus and 

 Ceratodus, irregular in Amia, Protopterus, Lepidosiren, and some Teleostei, 

 or else it is smooth. It derives its supply of blood either from the aorta or 

 caeliac artery in chondrostean Ganoidei, Lepidosteus, and Teleostei, or by two 

 pulmonary arteries arising, as in Amphibia, from the fourth branchial veins 

 (fourth aortic arches) in Polypterus, Amia, and Ceratodus, or by a single 

 artery from the common aortic root of the left side, produced by the union 

 of all the branchial veins, in Protopterus and Lepidosiren. Its blood is 

 returned by a pulmonary vein into the left division of the sinus venosus in 

 Dipnoi, into the systemic veins, either portal, hepatic, or cardinal in all other 

 Fish. Its capillaries form radiating tufts in the Cyprinoidei : vaso-ganglia, 

 i.e. bipolar retia, in many Teleosteans, e.g. Anguilla, Gadus, Perca. The 

 function of the sac in most instances is almost entirely hydrostatic ; hence 

 the name air- or swimming-bladder : but when the air within can be 

 renewed, as in Dipnoi especially, it must also act as a lung. But in all Fish, 

 when the branchiae are in full activity, it receives a supply of pure arterial 

 blood. 



In some Plectognathi, e.g. Diodon, an air sac opens into the ventral 

 side of the pharynx. It is distensible, and when inflated, these Fish float 

 on the surface, and are driven about by the currents. 



The heart lies in a pericardial cavity, shut off from the rest of the 

 coelome by an oblique or vertical fibrous septum. The two cavities com- 

 municate by a single canal, in Chitnaera (Holocephali), Acipenser, and 

 Spatularia, by a bifurcating canal in Elasmobranchii. The heart itself 

 consists of a sinus venosus, auricle, ventricle, and a tubular prolongation of 

 the latter containing several series of valves, the conus arteriosus, a structure 

 which is aborted in all Teleostei except in the Clupeid Butirinus (p. 88). 

 The sinus venosus is divided into a right systemic and a left pulmonary 

 section in Dipnoi. It opens into the auricle by an elongated aperture 

 guarded by two valve-like folds, or in bony Ganoidei and Dipnoi, by several 

 eminences. The auricle is thin-walled, its muscles disposed in interlacing 

 ridges, and its ventral wall coalesces with the conus in bony Ganoidei, less 

 completely in Dipnoi. In Lepidosiren its cavity is divided by a fenestrated 

 muscular septum ; by a dorsal fibrous ridge in other Dipnoi. There are no 

 auriculo-ventricular valves in this order : two valves with chordae tendineae 



