STOP OR CLICK MECHANISM IN THE ANIMAL KINGDOM. 209 



by the fact that 3 is very much bent inwards. (Compare 

 fig. 5, the head of the viper seen from below.) 



By exerting a pressure against the fang the members 

 3 and 3 of the right and left fang will consequently prees 

 ag-ainst each other and against the lower surface of the 

 skull. Thus the stress of the comparatively long bone- 

 stem 3 will be reduced. 



Besides this, there arises a considerable friction between 



3 and the skull which serves to fix the tooth. This friction 

 is all the greater, as there are layers of muscles between 1 and 

 3, which are sensitive. Besides, 3 is pressed against 1 through 

 the action of muscles. We therefore see that to fix the 

 fang the members 1, 2, 3 are of service and that the members 



4 and 5 are only of use in raising and lowering the tooth. 

 The running catch mechanism in two parts ^ of some fish 



appear to me to be more remarkable than these stopper-joints. 

 They consist only of two movable parts, the spine and bearer 

 of the spine. 



Double clicks in the arts are only used for locking, as in 

 the case of stove doors, but in this case one more often uses 

 screws now. The running mechanical stops are mostly in 

 three parts, because without a pawl they are not safe enough. 

 But the stops in fish radiants are mostly in two parts. Stops in 

 three parts are to be found, as far as I know, only in three 

 kinds of fish, but frictional stops in two parts are to be found 

 in many ; for instance, in the radiants of our small stickleback. 



The stickleback has in every case a sure protection in its 

 spines. Although it is so small the pike spares it on account 

 of its firmly set spines, and swallows without trouble the 

 larger carp although it has such sharp and tooth-like fins. 

 If the carp had fins like the stickleback, the pike would have 

 a bad time in the carp pond. 



Eeflecting on the life of the stickleback we see how much 

 he requires his spines. When the perch and the pike swallow 

 a stickleback it costs them their lives ; the salmon and the 

 haddock, however, swallow it without being harmed. The 

 greatest danger for it lies with the mother of its young ; they 

 are always on the alert to eat their children, and storm the 

 ^ Running and stationary catch mechanisms, see page 206. 



