500 ANATOMY OF VERTEBRATES. 



Polypterus 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-developement of gills on two 

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

 venous. But this difference relates only to the presence or 

 absence of a particular developement 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 chanrnno; the essential nature of the air-bladders themselves : 



o o 



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. 1 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 



O ' 



modifications of form or function of such air-bladders. Between 

 the completely divided air-bladder of the Polypterus and the un- 

 divided air-bladder of the Lepidosteus there are numerous degrees 

 of bifurcation in the series of fishes : it is to the undivided state 

 of the air-bladder in the Lepidosteus that its more strictly dorsal 

 position, and its communication with that aspect of the oesophagus, 

 are due : these modifications, however, do not affect its relation 

 of homology with the divided air-bladder of the allied genus 

 Polypterus, any more than with the divided air-bladders of the 

 Colitis larbatula or Arius gag or a, in which the divisions are con- 

 fined to the fore part of the abdomen, and are inclosed in osseous 

 cups developed from anterior trunk- vertebrae. Thus, the series 

 of transitions traceable in the air-bladders of fishes proves those 

 of the Lepidosiren to be the homologous organs ; whilst the 

 developement, relative position, and connection of the lungs of 

 the Batrachia equally prove those lungs to be the homologues of 

 the air-bladders of the Lepidosiren. Consequently, the air- 

 bladder of the Fish is homologous with the lungs of the Batrachian 

 and of all air-breathing Vertebrates ; although the air-bladder of 

 the fish does not perform the function of a lung, but is analo- 

 gous to the air-chambers in the Nautilus shell. 



87. Blood of Reptiles.- -The blood of Reptiles has red cor- 

 puscles of a flattened sub-biconvex elliptical shape ; proportionally 

 smallest in Ophidia, roundest in Chelonia, and largest in Batrachia : 



1 Compare xxxui. pi. xxvii. figs. 3 and 4, with xxv. pl.ii. figs. 5 and 6, and fig. 54, 

 xxxni. p. 182, with xxv. pi. ii. fig 7. 



