224 



OSTEICHTHYES 



below the oesophagus towards the right side (Parker [324], Spencer 

 [413]). Moreover, in the embryo, the bladder develops as a 

 median ventral outgrowth (Semon [399], Neumayer [308/)]). 



In all the Actinopterygii the air-bladder is essentially a median 

 dorsal organ, lying below the vertebral column and aorta and above 



Fig. i'.'7. 



Diagrams illustrating the blood-supply of the air-bladder in A. Polypterus, Ji. Ceratodus, C, 

 Amia, and D, a Teleost. The blood-vessels are seen from behind, and cut short in transverse 

 section, a, dorsal aorta; a.a.d, anterior dorsal artery from the coeliac; a.a.v, ant. ventral 

 artery ; a.b, air-bladder ; a.v.d, anterior dorsal vein to the cardinal ; &<J '', -4th aortic arch (6th 

 of the series); cv, coeliac artery ; </, ductus Cuvieri ; la, left pulmonary artery ; oe, oesophagus : 

 pr, portal vein receiving posterior vein from air-bladder; r.a, right 'pulmonary' artery; 

 r.ji. v, right (branch of) ' pulmonary ' vein ; i v, righl vein from air-bladder ; v, left ' pulmonary 

 vein. 



the alimentary canal, often outside the abdominal coelom. The 

 communicating ductus pneumaticus passes down the dorsal 

 mesentery to open into the oesophagus. The ductus is 

 short, and the opening wide in the more primitive forms (Lepi- 

 dosteus, Amia, Acipens&r). But although the bladder is dorsal in 

 the Actinopterygii, the opening of the ductus varies considerably 

 in position. In the Chondrostei, Amia, Lepidosteus, and the 

 majority of the Teleostei, it is quite or nearly median and dorsal ; 



