292 FISHES CHAP. 



necessarily involves a transit of air to and from that organ 

 through the ductus pneumaticus, and at present nothing is 

 known as to the method by which such inspiratory and expiratory 

 currents can be produced.^ 



There is also some experimental evidence for the belief that 

 the air-bladder of some Teleosts may be subsidiary to respiration 

 by acting as a reservoir for the superabundance of oxygen which 

 is taken into the blood through the gills, and subsequently re- 

 absorbed into the blood when the Fish is in water containing 

 relatively little oxygen.^ It is clear, however, that the conditions 

 under which the air-bladder can be used in this way are by no 

 means fully understood, for, under experiment, such Fishes died 

 of asphyxia even though after death the air-bladder still contained 

 upwards of fifty per cent of oxygen. 



Accessory Organs of Respiration. — In certain Fishes of 

 peculiar habits, or living under special external conditions, acces- 

 sory respiratory organs are developed. 



Although in this particular instance no special organs are 

 formed, mention may first be made of the singular method of 

 intestinal respiration in vogue in some Teleosts. In one of the 

 Loaches {Misgurnus fossilis)^ air is swallowed and passed along 

 the alimentary canal until it is finally voided at the anus. The 

 mucous membrane of the intestine is extremely vascular, and 

 hence the blood comes into sufficiently intimate relations with 

 the swallowed air to admit of it exchanging carbon dioxide for 

 oxygen. Intestinal respiration also occurs in species of the 

 South American freshwater genera of Siluridae and Loricariidae, 

 Callichthys, Boras, Loricaria, and Plecostomus ; ^ and in some 

 cases the area of respiratory surface is considerably increased by 

 the development of folds and processes of the intestinal mucous 

 membrane. 



In a few tropical Teleosts curious labyrinthiform organs are 

 developed in connexion with certain of the branchial arches, and 

 serve as accessory breathing organs. In the Indian " Climbing 

 Perch " {Anabas scandens),^ of the family Anabantidae, the organ 

 (Fig. 169) consists of three or more concentrically-arranged bony 



^ Sbrensen, Journ. Anat. and Phys. 1894, p. 127-138. 



^ Moreau, Ann. d. Sci. Nat. ser. 6, Zool. iv. 1876, Art. 8, p. 62. 



^ Erman, Gilbert Ann. d. Physik. xxx. 1808, p. 113. 



* Jobert, op. cit. ; iUd. v. 1877, Art. 8. 



° Zograff, Quart. J. Micr. Sci. xxviii. 1888, p. 501. 



