ITS BRISTLES FOR PROTRUDING. 18] 



drously curious. I will describe each of these in 

 turn. Behind the head, (or what for con-venience may 

 be so termed) the sides of the body are cut into nipple- 

 like feet, about seven pairs in all, which are perforated, 

 and carry so many bundles of fine, elastic, homybris 

 ties, like the hairs of a camel's-hair pencil, each pencil 

 carrying from twenty to thirty bristles. By means of 

 suitable muscles, the pencils are pushed out to their 

 full length, or withdrawn so as to be wholly sheathed 

 in the foot. 



Now let us look at the structure of these bristles. 

 A few are simple hairs, but the majority are instru- 

 ments of elaborate workmanship, though high powers 

 of the microscope are needful to display them well. 

 Each bristle consists of a transparent, yellow, horny 

 shaft, the extremity of which dilates ipto a slightly 

 enlarged knob. This is cleft into four points, three 

 of which are minute, but the fourth is developed into 

 a long, slightly divergent, highly elastic, tapering, and 

 finely pointed spear. 



These organs come into operation when the animal 

 would extrude its body from the mouth of its tube. 

 Their action is manifestly that of pushing against the 

 walls of the interior, which on close examination are 

 seen to be lined with a delicate membrane, exuded 

 from the animal's skin. The opposite feet of one 

 segment protrude the pencils of bristles, one on each 

 side, the acute points of which penetrate and catch in 

 the lining membrane ; the segments behind this are 

 now drawn up close, and extend their bristles ; these 

 catch in like manner ; then an elongating movement 

 takes place ; the pencils of the anterior segments 



R 



