360 



FISHES 



and relaxed in such a way as to initiate a series of vibratory 

 movements in the gases of that organ, and so produce definite 

 sounds. In not a few of the Fishes the cavity of the bladder 

 is subdivided by external constrictions or by internal septa, or 

 is complicated by the development of lateral, tubular, caecal 

 branches ; and hence the vibratory movements of the gases will be 

 greatly strengthened by their passage across the edges of the 

 septa, or the apertures of the caeca, and the intensity of the 



pt.i l.o m 



FIG. 207. Ventral view of the air-bladder and 

 its extrinsic muscle in Platystoma. a.b, 

 Air-bladder ; a.l.c, left antero-lateral caecum 

 of the bladder ; b.o, basioccipital ; b.w, body- 

 wall in contact with the lateral wall of the 

 bladder ; c 1 , centrum, of the first vertebra ; 

 cl, clavicle ; d.p, ductus pneumaticus ; m 1 

 and m 2 , extrinsic muscles of the bladder ; 

 pt.i, post-temporal. (From Bridge and 

 Haddon. ) 



FIG. 208. Air-bladder and its 

 muscles in Micropogon un- 

 dulatus. a. b, Air - bladder ; 

 Lb, right lateral caecum ; 

 m, m, musculo-tendinous ex- 

 tensions from the muscles of 

 the body-wall, which partially 

 invest the surface of the air- 

 bladder. (From Sorensen.) 



resultant sounds also increased. It will be readily understood 

 that the nature and quality of the sounds emitted by different 

 Fishes will necessarily vary with the shape of the air-bladder, 

 the number and arrangement of the internal septa and the caeca, 

 and the strength and disposition of the contracting muscles. 

 In a few Teleosts (Triglidae and Zeidae) sounds are said to be 

 produced by the rapid vibration of an annular, or centrally-per- 

 forated, muscular diaphragm, which stretches across the cavity of 

 the air-bladder. 1 Nevertheless, it must be strongly emphasised 

 that, while in some Fishes the air-bladder and its muscles 



1 Moreau, Compt. Eendus, lix. 1864, p. 436 ; Ann. d. Sci. Nat. (6) iv. 1876, 

 p. 65. 



