360 FISHES CHAP. 
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 
cl ] ae be Le 
Fic. 207.—Ventral view of the air-bladder and Fic. 208.—Air-bladder and its 
its extrinsic muscle in Platystoma. a.b, 
Air-bladder ; a./.c, left antero-lateral caecum 
of the bladder ; 6.0, basioccipital ; b.w, body- 
wall in contact with the lateral wall of the 
bladder ; c!, centrum of the first vertebra ; 
cl, clavicle; d.p, ductus pneumaticus ; m1 
muscles in Jicropogon wun- 
dulatus. a.b, Air- bladder ; 
lb, right lateral caecum ; 
m, m, musculo-tendinous ex- 
tensions from the muscles of 
the body-wall, which partially 
and m?, extrinsic muscles of the bladder ; invest the surface of the air- 
pt., post-temporal. (From Bridge and bladder. (From Sdrensen. ) 
Haddon. ) 
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."| Nevertheless, it must be strongly emphasised 
that, while in some Fishes the air-bladder and its muscles 
* Moreau, Compt. Rendus, lix. 1864, p. 436; Ann. d. Sei. Nat. (6) iv. 1876, 
p. 65. 
