worms. 265^ 



back, it follows that their teeth are inserted without any 

 protection, and that on account of their backward direction 

 they can be withdrawn only with great difficulty; thus, 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 the parts bear to weapons of human 

 construction. The manner in which they act as imple- 

 ments of locomotion has been beautifully demonstrated 

 by Dr. Williams in the ISTereidous 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 mechanical prin- 

 ciples, nothing cau 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 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 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 in the organisation of the Annelida, in which 

 Nature takes adroit advantage of mechanical principles 

 in the attainment of her ends." t 

 * " Litt. de la France," ii. 71. + " Kep. on Brit. Annelida," 211.. 



