54 MB. E. H. BTJENE OK THE AORTIC-AECH 



cucMa*, the two respiratory bladders on either side of the neck 

 obtain tbeir blood-supply from the first pair o£ branchial aortic 

 arches. 



la looking through the above list, one is at once struck by the 

 frequency with which the pulmonary artery is derived from 

 the fourth aortic arch ; and this not only when the lungs are in 

 all probability homologous structures, but in creatures having 

 different kinds of air-breathing organs, some of which can bear 

 no morphological relationship to each other. 



There certainly are exceptions, but they are comparatively 

 very few ; in fact, even including Lepidosiren, which, from the 

 compressed condition of its branchial apparatus, ought scarcely to 

 be used as an argument either way, they only amount to three ; 

 and even of these three we have seen that one, i. e. Saccohranchus, 

 is sometimes found in what may be called the normal condition. 



The general tendency appears to be that any organs modified to 

 act as lungs, no matter what may be their morphological cha- 

 racters, are supplied with blood by the fourth branchial aortic 

 arch. In the higher Yertebrata this is the case without exception ; 

 and even among fishes, where presumably the organ specialized 

 for breathing air is not so firmly established, this is still the 

 case, although liable to variation. 



SacGohrancJius and Amphipnous agree in respect to the origin 

 of their afi'erent pulmonary vessel from the afferent branchial 

 system ; and therefore it is specially interesting to note that 

 S. singio is abnormal in the partial realization of that character 

 (origin of pulmonary artery from the first branchial arch) which 

 is diagnostic of Aonphipnous. 



It has been suggested to me by my friend and late teacher 

 Prof. Howes, that the variations occurring in the pulmonary 

 artery of these fishes may find a parallel in the variability which 

 he observed iu the first appearances of the epiglottis in the 

 Amphibia *. So far as I am aware, this may very well be the 

 case, since there appears to be considerable liability to variation 

 in organs that are in the initial stages of their development, and, 

 so to speak, still on their trial. 



* Hyrtl, " Ueber den Amphibienkreislauf von Amphipoious und Monopterus" 

 Denkschr. Akad. Wiss. Bd. xiv. 1857, p. 39. 



t Gr. B. Howes, " On a hitherto unrecognized Feature in the Larynx of the 

 Anurous Amphibia," P. Z. S. 1887, p. 491. 



