POLYPTERUS 



367 



Ob.C. 



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co.c. 



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T.l. 



a,.c. 



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relatively very dense and heavy scales and where the rest of the bony 

 skeleton is also very dense, this action must be peculiarly important. 

 But this floating effect necessitates that the lung apparatus as a whole 

 must be approximately symmetrical about the mesial plane as otherwise 

 the creature would be tilted over on one side. Now in Polypterus the 

 original symmetry is lost through the relatively greater increase in. the 

 size of the right lung. As a corrective to this the hinder portion of the 

 right lungj having no , 



left lung to balance it, 

 loses its right-hand posi- 

 tion and grows back 

 along the mid - dorsal 

 line (Fig. 153, r.l). This 

 hinder portion of the 

 right lung then lies 

 mesially, while the front 

 portion, balanced by the 

 small left lung, retains 

 its primitive right-hand 

 position. It is clear that 

 if the left lung were to 

 continue to shrink still 

 further in relative size, 

 to the verge of complete 

 disappearance, a larger 

 and larger proportion of 

 the right lung would 

 take up a mid-dorsal 

 position until the whole 

 became mesial. A con- 

 dition would thus be 

 reached identical with 

 that of the air-bladder of a teleost except that the air-bladder would 

 still communicate round the right side of the alimentary canal with 

 a ventrally placed glottis. That this reasoning is correct is indicated 

 by one of the lung-fishes {Ceratodus, p. 375) which actually possesses 

 such an arrangement (Fig. 154, C). To attain to the arrangement of a 

 physostomatous teleost all that is now needed is for the communication 

 between air-bladder and glottis to become shortened, as on the principle 

 of economy of tissue — one of the great principles of animal development 

 — we should expect it to do. For this would involve the glottis becoming 



B 



Diagram illustrating the lung in Fishes ; as seen from the left 

 side. A, Primitive symmetrical arrangement ; B, Polypterus ; 

 C, Ceratodus ; D, physostomatous Teleost; E, physoclistic Teleost. 

 a.c. Alimentary canal ; g, glottis ; l.l, left lung ; r.l, right lung. 



