A DEGENERATE GLAND. — PRINCE. 223 



Siluroids, and Carps ; but it has never been demonstrated by 

 experiment that these tishes hav^e hearing or sound-producing 

 powers superior to other fishes not possessed of this apparatus. 

 The sturgeon and cod, though provided with a large swim- 

 bladder are silent. If the swim-bladder, again, be barometric 

 why is it denied to so many fishes to which a knowledge is 

 necessary of the variations in pressure of their atmospheric and 

 aqueous surroundings ? The presence of the organ in difi^erent 

 families of fishes, as Gegenbaur points out, indicates some 

 important purpose, though it may be said that the persistence of 

 numerous obsolete and disused organs is most remarkable, the 

 thymus gland being a familiar example. An external or exposed 

 organ, which becomes useless may disappear rapidly like the pec- 

 toral fin of the lower side in the variegated sole (Solea variegata, 

 Donov.), but an internal deep-seated structure is more likely to 

 remain and become modified rather than disappear. Such 

 transformation and change of function the swim-bladder of 

 fishes has unquestionally undergone. 



Literature referred to. 



1. Boulenger, G. A. Brit. Museum Guide to Fishes, &c. London, 1898. 



2. Borelli. De Motu Animalium. Rome, IBsO. 



3. Carpenter, Dr. W, B. Human Pinysiology, 9th Ed. London, 1881. 



4. Challenger Reports. Vol I, Phys. and Chem. 



5. Cleland, Prof. John. Memoirs in Anatomy. Glasgow, 1889. 



6. Delaroche, F. Ann. du Mus. dHist Nat., t. XIV. 



7. Dufosse. Ann d. Sci., Nat., V. Ser., t , 19, 20. 1874. 



8. Gegenbaur, Prof. Carl. Elem. of Comp. Anat. London, 1878. 



9. GiJnther, Dr. A. C. L. G. Introd. to Study of Fishes. Edinburgh, 1880. 

 9a. Hamilton, Dr. W. R. Croaking of the Perch. Bull. U. S. F. Coram , 



Vol. VII. 1887. 



10. Holt , E. W. L. Eggs and Larva; of Teleo?teans. R. S Dublin, Vol. IV, 

 Ser. 2. 1891. 



11. Jordan and Evermann. Amer. Food and Game Fislies. 1902. 



12. Krause, Dr. AUgemeine Anatom 1876. 



13. Mcintosh and Prince. Life Histories of Food Fishes. Trans. R. S. 

 Edin., XXXV, p. iii. 1890. 



14 Macalister, Prof. Alex. Introd. to System. Zool. of Vertebrate 

 Animals. Dublin, 1878. 



