302 EVENINGS AT THE MICROSCOPE 



in this I think they are in error, misled by the resem- 

 blance, already alluded to, which they bear to weapons 

 of human construction. The manner in which they act 

 as implements of locomotion has been beautifully dem- 

 onstrated by Dr. Williams in the Nereidous "Worms, of 

 which he observes that in nearly all species the feet are 

 constructed with express reference to progression on solid 

 surfaces. In many instances, the bristle is compound, 

 consisting of a staff with a variously armed point or blade 

 jointed to its extremity. "Viewed by the light of mechan- 

 ical principles, nothing can be so obvious as the reason 

 why the setce in these, as in nearly all other Annelida, are 

 jointed. If they consisted of rigid, unbending levers, it 

 is manifest that they would prove most awkward additions 

 to the sides of the animals ; if fixed too deeply in the sur- 

 rounding soil, they would not act at all as levers; if too 

 superficially, the Worm would be compressed in its tube 

 at the moment when the setce of the opposite feet would 

 meet in a straight line. These difficulties are effectually 

 and skilfully obviated by the introduction of a joint or a 

 point of motion on each seta. This is one instance among 

 many which the eye of the mechanician would detect on 

 the organization of the Annelida in which Nature takes 

 adroit advantage of mechanical principles in the attain- 

 ment of her ends." ' 



Look now, in illustration of these principles, at the 

 bristle-feet of this beautiful green Phyllodoce. No doubt 

 you have often seen it in the little hollows of our rocky 

 ledges, and especially on beds of young muscles, and prob- 

 ably you have admired the elegant ease with which its 

 lithe and tortuous body writhes and winds, like a bit of 



1 "Rep. on Brit. Annelidse," 211. 



