4o8 FISHES 



of their bodies in their natural medium, namely water, but that 

 the vibrations originate in connection with a gaseous medium 

 and in fact are produced by the organ which is homologous 

 with the lungs, by which the voice of terrestrial vertebrates is 

 produced. There are, however, cases in which sound is pro- 

 duced, as in other aquatic animals, by structures entirely sur- 

 rounded by water ; this mode of producing sound is called 

 stridulation, and stridulating organs occur in many Teleosteans. 

 One of the most complicated organs of this kind is that of the 

 Indian siluroid, Callomystax gagata ; in this fish the dorsal 

 spines of the third, fourth and fifth vertebrae are united together 

 and articulated with the supraoccipital bone of the skull ; the 

 united spines form a high narrow plate of bone which is vertic- 

 ally divided behind by a cleft in which moves the first inter- 

 spinous bone of the dorsal fin ; the inner sides of the cleft and 

 the outer sides of the interspinous bone are covered with close- 

 set vertical ridges like those of a file, and these ridges rubbing 

 against each other produce a harsh grating noise ; the fifth and 

 sixth vertebrae are joined by an unusually wide intervertebral 

 ligament so that they can move upon one another freely and 

 the stridulating organ is put into action by the bending of this 

 joint up and down in the vertical plane, the ordinary lateral 

 movements of the body having no effect upon it. Stridulation 

 between different parts of the skeleton has been stated to occur 

 in many species of fish and is probably used as an expression 

 of emotion, but owing to the natural difficulties evidence of the 

 actual and voluntary production of sound by the fish in its 

 natural state is scanty or wanting. In some siluroids a pro- 

 cess of the anterior spine of the pectoral fin stridulates against 

 the wall of a socket in the pectoral girdle to which the fin is 

 articulated. The spines of the dorsal fin are said to produce 

 sound by friction against neighbouring bones in several trigger- 

 fishes of the genera Batistes, Monacantkus, and Triacanthus 

 in the boar-fish {Capros aper), and in the three-spined stickle- 

 back. According to Dr. Otto Thilo, however, the peculiar arti- 

 culations of the spines in these cases are examples of click 

 mechanisms, the effect of which is to keep the spines erected 

 without muscular exertion, as they can only be depressed when 

 the click is removed, as in the mechanism of a clasp knife ; such 

 an arrangement probably produces a sound, at any rate in the 



