WORMS. 265 



they can be withdrawn only with great difficulty ; thus, in 

 most cases, the dart becomes broken ; but the animal is 

 furnished with so great a number, that these losses are 

 scarcely felt, and there remain to it amply sufficient for its 

 defence in all contingencies." * 



You will have noticed that the learned French zoologists 

 seriously countenance the notion that these exquisitely 

 elaborate organs are weapons of offence. But in this I 

 think they are in error, misled by the resemblance, already 

 alluded to, which they bear to weapons of human construc- 

 tion. The manner in which they act as implements of 

 locomotion has been beautifully demonstrated by Dr. Wil- 

 liams 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 in- 

 stances, the bristle is compound, consisting of a staff with 

 a variously armed point or blade jointed to its extremity. 

 " Viewed by the light of mechanical principles, nothing 

 can be so obvious as the reason why the seta in these, as 

 in nearly all other Annelida, are jointed. If they con- 

 sisted 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 surrounding 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 seta of the opposite feet would meet in a straight 

 line. These difficulties are effectually and skilfully ob- 

 viated 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 in the organisa- 

 tion of the Annelida, in which Nature takes adroit ad- 

 vantage of mechanical principles in the attainment of her 

 ends." f 



Look now, in illustration of these principles, at the 

 bristle-feet of this beautiful green Phyllodoce. No doubt 

 * Litt. de la France," ii. 71. f " Rep. on Brit. Annelida," 211. 



