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 Avhich 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. 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 

 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 

 Cobitis barbatula or Arius gagora, in which the divisions are con- 

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

 cups developed from anterior trunk-vertebra?. 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 Avith 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 xxxm. pl.xxvii. figs. 3 and 4, with xxv. pi. ii- rigs. 5 and 6, and fig. 54, 

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



