SYSTEM OF SACCOBEANCHUS FOSSILIS. 49 



wandering far from any water, stirred the curiosity of naturalists, 

 and finally led to the recognition, as accessory respiratory organs, 

 of certain structural modifications occurring in these fishes. 



For some time the exact method by which this respiration was 

 effected remained doubtful. However, during the last twenty-fire 

 or thirty years numerous interesting experiments have been per- 

 formed by Day and others * upon most of these Indian freshwater 

 fishes, which tend to prove that the modifications in the pha- 

 ryngeal region of these creatures (epibranchial and other organs) 

 do not contain water for moistening the gills as was originally 

 supposed, but air for purposes of direct aerial respiration t. In 

 certain other air-breathing fishes, i. e. the bony Ganoids and the 

 Dipnoi, the same end is attained by a modification of the swim- 

 bladder. 



Further details upon this subject are unnecessary, as my 

 object is merely to draw attention to the fact that among fishes 

 bearing no close relationship to each other there are to be found 

 specialized organs diff"ering in their morphological characters, 

 but which are all, physiologically speaking, lungs. 



In the East-Indian rivers there is to be found a curious air- 

 breathing Siluroid, byname Saccohranchus, in which the accessory 

 respiratory organ takes the form of a pair of long narrow air- 

 pouches, which lie along the back on either side of the vertebral 

 column above the transverse processes, and extend for three 

 parts the length of the fish, from the branchial chamber to within 

 four inches of the tail. Yenous blood is conveyed directly from 

 the heart to the air-sacs by branches of a pair of the aff'erent 

 branchial arteries, and returned, after oxygenation, into the 

 aorta. 



Hyrtl J, who has worked out the anatomy of this fish, describes 

 the arrangement of the branchial arteries with reference to the 

 air-sacs as follows : — "The fourth left branchial artery surpasses 



* Day, Proc. Zool. Soc. 1868, p. 274, and Journ. Linn. Soc. (Zool.) vol. xiii. 

 p. 198 ; Dobson, Proc. Zool. Soc. 1874, p. 312. 



t From his researches on the blood-supply to the supra-branchial chamber 

 in the Ophiocephalida;, Hyrtl considers that this organ is not for breathing 

 air, but is probably a water-reservoir for moistening the gills. (Hyrtl, " Ueber 

 das Labyrinth und die Aortenbogen der Gattung Ophiocephalus" Sitz. Akad. 

 Wiss. Bd. X. 1853, p. 148.) 



I Hyrtl, " Zur Anatomic von Saccobranchus singio" Sitz. Akad. Wiss. 1853, 

 Bd. xi. Heft 1, p. 302. 



L12^']S^ JOUKN — ZOOLOGY, VOJ,. XXV. -J- 



