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.—JIn 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, Doras, 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 
1 Sorensen, Journ. Anat. and Phys. 1894, p. 127-138. 
2 Moreau, Ann. d. Sci. Nat. sér. 6, Zool. iv. 1876, Art. 8, p. 62. 
3 Erman, Gilbert Ann. d. Physik. xxx. 1808, p, 113. 
4 Jobert, op. cit.; ibid. v. 1877, Art. 8. 
5 Zograff, Quart. J. Micr. Sci. xxviii. 1888, p. 501. 
