100 Miscellaneous. 



employed. In the Siluroids the unossified portion of the swimming- 

 bladder is slightly elastic, in a nearly equal manner throughout its 

 whole extent ; whilst in the Characini the elasticity depends espe- 

 cially upon flat bands or round cords in the wall. The swimming- 

 bladder acquires its greatest development as an organ of sound in 

 the Siluroids. In the species of the genera Platystoma and Pseuda- 

 roides it is divided by a longitudinal partition and by several trans- 

 verse partitions into a number of chambers or cells, which, however, 

 freely communicate with each other. In the genus Doras the 

 swimming-bladder presents numerous diverticula, divided internally 

 by incomplete septa into a great number of small cells. In all these 

 fishes the transverse apophyses* of the two or three first vertebras, 

 and frequently a part of the arch of the first vertebra, are not only 

 bound to each other, but also to the posterior part of the cranium 

 and the apophyses of the first vertebra, by very strong elastic mem- 

 branes. The transverse apophyses of the second or third vertebra, and 

 sometimes of both these vertebras, are shaped into the form of very 

 powerful springs t, and closely united to the swimming-bladder. 

 The sound is produced by the action of the muscles which are 

 inserted either directly in the swimming-bladder or upon the trans- 

 verse apophyses of the third vertebra. In the Characini the elastic 

 parts of the swimming-bladder are stretched in the direction of their 

 length by the contraction of the muscles ; and the vibration that 

 results from this rhythmic movement is transmitted to the air con- 

 tained in the cavity of the swimming-bladder. In the Siluroids the 

 anterior portion of the swimming-bladder is drawn alternately 

 forward and backward by the contraction and relaxation of the 

 muscles. During these movements the air, in passing across the 

 incomplete transverse septa, sets the latter in vibration, and the 

 sound is produced. The height or rather the depth of the sound is 

 in direct proportion to the rapidity of the vibrations of the springs. 

 The fishes which I have been able to study in this connexion 

 belong to the genera Ageniosits, Doras, Platystoma, Pseudaroides, 

 Procliilodus, Chalcinus, Pygocentrus, and Myletes ; several of them 

 were observed living. None of these fishes respire by means of the 

 swimming-bladder. A memoir containing the details of these obser- 

 vations will appear in the ' Naturhistorisk Tidsskrift ' of Copen- 

 hagen. — Comptes Rendus, May 19, 1879, p. 1042. 



* According to Weber (' De auri et auditu hominis et aninialinm,' 

 p. 1, Lipsise, 1820) these apophyses, in the genus Silurus and in the 

 species of the family Cyprinoidei, serve to put the ear in communication 

 with the swimming-bladder. I cannot say whether there is an organ of 

 sound in the latter family, not having yet examined them in this respect. 



t According to John Midler (' Archiv fur Anat. und Physiol ogie,' 

 1842, p. 319), this apparatus, in the genera Auc/ieniptents, Si/)iodontns, 

 Doras, Malacopterurus, and Euanemus, assists in locomotion by conden- 

 sing or rarefying the air in the swimming-bladder. Upon this interpre- 

 tation M. Moreau, in his excellent memoir on the swimming-bladder 

 (Ann. Sci. Nat. 6 e ser.iv. Art. no. 8), has already expressed certain doubts, 

 the justice of which I am in a position to prove completely. 



