THE TRIGGER-FISH 



The reason for this is that these rays have a ball and 

 socket articulation like those of the limbs of the higher 

 vertebrates and they are furnished with certain refine- 

 ments which facilitate their action. They are attached 

 to a basal bony, keel-shaped structure, lodged in the 

 tissues of the back. The first ray is jointed to this by 

 means of a two-lobed head and above them, on its 

 hinder face, there is a depression turned towards the 

 second ray. This also has an articular head with two 

 lobes, but these, stretched out and extended like a 

 pair of tongs, overlap the basal piece, and can slip 

 along horizontally. Further, this head has on its front 

 surface, turned towards the first ray, a projection which 

 fits into the hollow with which this is furnished. 

 Now we can understand the mechanism. When the 

 fin is set up, the second ray slides forward and comes 

 closer to the first, fitting its projection into the depres- 

 sion just as a bolt slides into its socket. It is impossible, 

 so long as this bolt is shot, to pull down the first ray. 

 In order to do so it is necessary to press upon the 

 second, slide it back, and so bring out the projection 

 from the depression, or the bolt from the socket. 

 Then the first ray, no longer having anything to 

 support it, may easily be brought down, bringing the 

 whole fin with it. As for the third and last ray, a 

 ligament connecting its base with that of the second 

 makes it possible to act upon the latter from a distance, 

 make it slide, and so lower the fin. 



In vertebrates, the bony skeleton is a complicated 

 piece of apparatus, made up of a number of pieces, 

 which sometimes support one another in such a way 

 that they hold together like parts of a framework, 

 sometimes move one over another to allow movements 

 of various parts of the body. In fishes, the supporting 

 articulations are incapable of movement, or nearly so, 

 and they are the commonest, often the only, articula- 

 tions. The body is all of a piece; the movements 

 are limited in extent. This is the very opposite of 



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