WORMS. 305 



the dart becomes broken ; but the animal is fui-nished 

 with so great a number, that these losses are scarcely 

 felt, and there remain to it amply sufficient for its de- 

 fence in all contingences." * 



You will have noticed that the learned French zool- 

 ogists seriously countenance the notion tliat these ex- 

 quisitely elaborated oi-gans arc 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 liuman construction. The manner in which 

 they act as implements of locomotion has been beauti- 

 fully demonstrated by Dr. "Williams in the Kereidoua 

 Worms, of which he observes that in nearly all species 

 the feet are constructed with express reference to pro- 

 gression on solid surfaces. In many instances, the 

 bristle is compound, consisting of a staff with a vari- 

 ously armed point or blade jointed to its extremity, 

 " Yiewed by the light of mechanical principles, nothing 

 can be so obvious as the reason whv the setw in these, 

 as in nearly all other Annelida, are jointed. K 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 super- 

 ficially, the "Worm would be compressed in its tube at 

 the moment when the setcB of the opposite feet would 

 meet in a straight line. Tliese difficulties are effectually 

 and skilfully obviated by the introduction of a joint oi 

 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 



* Litt. de la France, ii. 1\. 



