STOP OR CLICK MECHANISM IN THE ANIMAL KINGDOM. 211 



In the joints of the rays of the stickleback and some 

 shad-fish, these props are distinctly shown, because only the 

 upper part of the sheath in which the sword-formed points 

 of the fin rays lie is bone. The under part of the sheath 

 is of a soft sinewy covering. 



Should the reader have the opportunity of having a stickle- 

 back in his hand, he will, after these explanations, have no 

 difficulty in laying down the upright fin with the help of a 

 pin-point, as he knows the spot on the joint of the spine where 

 he must press. 



The click mechanism of the ventral fin of Triacanthus (fig. 8) 

 is quite different from those already described. On the back 

 side of the ventral fin is to be found a spur-like bone 

 elongation (fig. 3, click elongation). If one moves the fin 

 backwards and forwards this elongation slips up and dowfi. 

 This bony wall stands in a sloping direction so that the 

 elongation slips down when one raises the fin; on the other 

 hand, it must slip up when the fin has to be laid down, therefore 

 one can only lay it when one turns it round its longitudinal 

 axis so that the elongation is raised from the bony wall. It 

 resembles a door standing over a sloping floor ; when such a 

 door has sunk it is only possible to raise it by its hinges. 



The spine of the three-spiked Triacanthus shows, again, a third 

 kind of double friction stop. Behind the dorsal spine of the 

 fish there is a small sharp bone continuation (fig. 9, side 

 view) which fits precisely into a wedge-shaped opening. In 

 fig. 9 the opening is shown by means of the removal of the 

 side bones. The raised spine can therefore only be lowered 

 when it moves exactly in its plane of revolution. Only the 

 slightest side movement is necessary to fix it. 



These running catch mechanisms in two parts form the 

 three principal kinds of stops which are found in fish fins. In 

 these are found very many deviations that often are so im- 

 portant as to be with difficulty referred to one of the primary 

 forms ; but these deviations are of importance to the natural 

 philosopher and the mechanician. The natural philosopher 

 gains through them an understanding of many differences 

 of form which were formerly for him a closed book. He is 

 in a position to give a mathematical explanation why certain 



