STOP OR CLICK MECHANISM IN THE ANIMAL KINGDOM. 207 



are of little importance in comparison to the click mechanism 

 which makes it possible for a fish to hold up its fins without 

 any muscle power. 



These contrivances are frictional stops, which consist either of 

 two or three movable parts. 



The frictional stops in fish in three parts remind one 

 much of the runuinc' catch mechanism which is so much 

 made use of in practical mechanics. Already Eeuleaux has 

 remarked on the great resemblance, and he said in his 

 Kronstructeur (Braunschweig, 1895) : " It may be remarked that 

 frictional stops also appear in nature." Some kinds of fish 

 fix, by means of a stop in three parts, certain bone formations 

 (fins) upright, and can also lay them down (see Thilo, Die 

 Spurgeleiike einiger Wehe, etc. Dorpat, 1879). 



The well-known Dobuschen frictional feeds and the American 

 Yale-locks contain catch mechanisms identical with those which 

 we find in the spinal fins of certain fish which go by the name 

 Monacanthus, which live in the coral reefs of the Red Sea 

 (fig. 11). 



The Arabian fisher boys know the Monacanthus very well. 

 They have often experienced that these fish retreat into the 

 holes of the rocks, and raise their fins against the roof of the 

 holes. It is only then possible to get them out by laying 

 down the spiked fin, but that can only be done by pushing down 

 the small spike behind the spiked fin, as by the lock of a gun. 

 In fig. 11 this second spike is shown as a click bone pushed 

 back. One sees at once that it is equivalent to the mechanism 

 in the Yale-lock, fig. 13. 



The frictional path of the mechanism has a circular profile, 

 whereas the frictional path of the click bone shows an involute 

 profile. This involute must be calculated from a very small 

 primary circle, the middle point being in M, fig. 13. 



In the Yale-lock the circular profile is enough, as the bolt 

 has a straight course. By the click bone the involute profile 

 is essential, because the strengthened part of the fins shows 

 a circidar movement. In my models of the cHcks of the 

 Monacanthus,! I had to choose the involute profile, because 



1 Haferland and Pippow sell these models. Berlin : N. Hermsdorf 

 Bahnhofstr. Villa, No. 5. 



VOL. XXXV. (N.S. vol. XV.)— JAN. 190L 



