206 DR OTTO THILO. 



bouy skeletons. On these I may be allowed to particularise, 

 as I have studied and .written about some of them.^ 



The object of these stop mechanisms in solid bodies is to save 

 muscle strength. Particularly one sees it on the fin rays of the 

 little fish called the stickleback. 



When one observes how this little fish holds up its fin 

 rays for hours together, while it watches over the nest of its 

 young brood, we ask how the fin muscles stand the unremitting 

 fatigue. On examining minutely the joints of the fin rays, 

 I found that the stickleback held its fins upright without 

 muscular exertion simply by a stop or click mechanism. 

 Also on a dead stickleback it is only possible to depress the 

 spines when one understands how to unlock the mechanism. 



Similar contrivances are to be found in many other kinds of 

 fish, and the diversities which we see correspond exactly to the 

 object which they have to serve. 



The perch (fig. 1, Plate XXV.), for instance, moves the fin 

 rays of the dorsal fin backwards and forwards in swimming, 

 and cannot hold them upright for a length of time like the 

 stickleback. Its fin joints are built more for movement than 

 for fixity, that is, the contrivance by which the perch helps to 

 hold up the fins is less developed than that of the stickleback. 



If one raises the fin rays of a perch, one notices that the 

 first spine is very much bent forward, probably because it is 

 easier for the muscles to hold the leaning rays against the 

 water stream in swimming than if they were perpendicularly 

 fixed. For the same reason the masts of a ship are fixed 

 backwards on purpose that the strain on the ropes may be less 

 when the ship sails before the wind. 



By some kinds of fish the first spine takes almo.st a horizontal 

 position if the fin is quite raised. One can, on this account, say 

 the spine stands in this position on a dead centre against the 

 water stream against which the fish swims. 



By other sorts of fish it is again a peculiar fixing of the joint 

 axis and a particular asymmetrical build of the joint bodies which 

 help the fish to hold the fins upright. 



Now, all these methods whereby the fins are held upright 



1 Vide Dr Otto Thilo, Die Unibildungm an den Gliedmasseii dcr Fische, 

 Leipzig, Engelinann, 1896. 



