AIR-BLADDER 223 



The ossification of the endoskeleton, so characteristic of these fish, 

 has already been dealt with in a general way above (p. 60). Most 

 Teleostomes have a number of endochondral bones in the cranium 

 and visceral arches, which are comparable with those found in the 

 terrestrial vertebrates ; but since they are mostly absent from the 

 modern Dipnoi, and many of them have not yet been shown to 

 occur in extinct Dipnoi, it cannot be asserted that they were 

 present in primitive fish ancestral to these sub -classes. The 

 description of these bones of the endoskeleton may therefore be 

 left till later (p. 266). We may mention, however, that ventral (or 

 pleural) ribs are present throughout the Osteichthyes. 



In all the Osteichthyes an operculum is present on the hyoid 

 arch. It covers the branchial apertures, never more than five in 

 number, and is supported by bones the largest of which, the 

 opercular, is articulated to the hyomandibular. 



The air-bladder is one of those important structures which 

 distinguish the Osteichthyes from the Chondrichthyes. Some kind 

 of air-bladder is found in all the divisions of the former group, and 

 it can hardly be doubted that it is only absent in those Teleostomes 

 in which it has been lost. But in size, structure, disposition, and 

 blood-supply the air-bladder is so variable, that it is by no means 

 certainly homologous throughout the Osteichthyes. 



The air-bladder always develops as a diverticulum of the 

 alimentary canal generally from the oesophagus, but sometimes 

 farther back from the stomach. In Polypterus it is ventral and 

 bilobed, lying below the alimentary canal (Fig. 197, A) ; the large left 

 and smaller right lobe unite, and open through a muscular vestibule, 

 .or rudimentary 'larynx,' by a median ventral aperture into the 

 oesophagus far forward. A somewhat similar muscular vestibule 

 opens ventrally into the oesophagus (either in the middle line or 

 slightly to the right) in the Dipnoi. But here the vestibule leads 

 into a ductus pneumaticus which passes round the right side of the 

 alimentary canal to expand dorsally into the air-bladder, a median 

 (Ceratodus, Fig. 197, B), or bilobed sac (Protopterus, Fig. 198). 

 Blood is supplied to the bladder in Polypterus and in the Dipnoi by 

 paired afferent ' pulmonary arteries,' derived from the last (fourth) 

 branchial arch, the sixth of the embryonic series (Fig. 197). The 

 blood is returned to the heart in Polypterus by paired efferent vessels, 

 opening into the hepatic vein near the sinus venosus ; in the Dipnoi 

 by paired vessels uniting and passing directly to the sinus venosus 

 itself on the left side. That the air-bladder of the Dipnoi was 

 originally ventral, and that its dorsal position has been secondarily 

 acquired, is clearly shown by the course of the ductus pneumaticus, 

 jand of the left afferent and the left efferent vessels which pass round 



