280 LECTURE XI. 



scaly integuments, the mucous tubes and pores on the head, the 

 ' lateral line,' and, in short, the totality of the organisation of the 

 Lepidosiren, will be deemed to fully prove its true ichthyic 

 nature. It is extremely interesting to find the Ganoid Polypterus, 

 which of all osseous fishes most closely resembles the Lepidosiren 

 in its spiral intestinal valve, in the bipartition of the long air- 

 bladder, the origin of the arteries of that part, and the place and 

 laryngeal mode of communication of the short and wide air-duct 

 or windpipe, also presenting the closest agreement with the Lepi- 

 dosiren in the important character of the form of the brain. The 

 common objection to the view of the air-bladder of fishes being 

 the rudimental homologue of the lungs of air-breathing Vertebrates 

 has been, that the artery of the air-bladder carries arterial blood, 

 that of the lungs venous blood. Let us compare the air-bladders 

 of the Polypterus and Lepidosiren in reference to this character. 

 The arteries of both are derived from the returning dorsal portions 

 of the branchial vascular arches before their union to form the aorta. 

 Li the Polypterus, according to Miiller, the artery of each air-sac is 

 formed by the union of the efferent vessels of the last gill : the blood 

 is, therefore, arterialised before entering the artery of the air-sac. 

 In the Lepidosiren, by reason of the non-developmeut of gills on two 

 of the branchial arches, the blood transmitted to the air-sac is 

 venous. But this diiference relates only to the presence or absence 

 of a particular development of the branchial vascular arches, from 

 which the air-bladders of the two species are supplied with blood : it 

 is a difference which modifies the function without at all changing 

 the essential nature of the air-bladders themselves : the relative 

 position of these vascular sacs, their form and size, their mode of 

 communication with the oesophagus, — in short, every character by 

 which relations of homology are determined, — are the same in 

 both Polypterus and Lepidosiren.* The lungs of the Lepidosiren 

 being, then, unequivocally the homologues of the air-bladder of 

 the Polypterus, it follows that they must be homologous with the air- 

 bladders of other fishes, whatever be the modifications of form or 

 function of such air-bladders. Between the completely divided air- 

 bladder of the Polypterus and the undivided air-bladder of Lepi- 

 dosteus there are numerous degrees of bifurcation in the series of 

 fishes : it is to the undivided state of the air-bladder in the Lepi- 

 dosteus that its more strictly dorsal position, and its communication 

 with that aspect of the oesophagus, are due : these modifications, 



* Compare xxxiii. pi. xxvii. figs. 3 and 4. with xxv. pi. ii. figs. 5 and 6., and 

 fig. 54. XXXIII. p. 182. with xxv. pi. ii. fig. 7. 



