94 



ANNELIDES. 



the little gutters in which the barbules are laid, and which are accu- 

 rately adapted for their reception. The end of this singularly beau- 

 tiful and complicated apparatus is obvious enough. Like the bristles 

 of the Annelidans in general, the lance-bristle is retractile ; but as 

 the barbules are pointed backwards, it could not have been drawn 



No. XVII. 



within the soft body without a painful laceration of the parts. To 

 obviate this, a sheath has been provided to enclose the whole ; and 

 thus covered, the bristle moves with an easy play. Nor is it difficult 

 to explain how, when the barbed point is needed by the necessities 

 of the worm, the sheath is put aside. This consists of two pieces or 

 valves accurately adapted to each other, leaving a small fissure or 

 gape where they meet above, and through which the apex of the 

 lance is occasionally seen protruded. The sheath is also flexible and 

 elastic, and its valves, which are naturally in close opposition, are 

 capable of easy separation. Thus, when an enemy or prey comes in 

 contact with the apex of the lance, this is thrust through the gape 

 of the valves onwards to penetrate the opposing object ; and by the 

 protrusion, the sheath, having lost the support it had from the lance, 

 is not only separated into its valves, but these are bent and folded 

 backwards. They return again to their natural position and closure 

 as the lance is withdrawn, and this is effected by their inherent elas- 

 ticity*. 



Other forms of Bristles will be described as they occur to us in 

 our examination of the species. In the meantime, as we are dealing 

 in marvels, let me ask the naturahst to count the number which may 

 be required to furnish the garniture of a single individual. There 

 are Annelides which have 500 feet on each side, — each foot has two 

 branches, — and each branch has at least one spine and one brush of 



* " In human workB, though labour'd on with pain, 

 A thousa]id movements scarce one purpose gain : 



n God's, one single can its end produce ; 

 Yet serves to second too some other use." 



Pope, Essay on Man, Epist. 



1. 53. 



