174 ITS BRISTLES FOR PROTRUDING. 



body from its shelly tube, and by wliicli it withdraws 

 on alarm with such inconceivable rapidity, is won- 

 drously curious. I will describe each of these in 

 turn. Behind the head (or what for convenience 

 may be so named), 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, 

 horny bristles, 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 out to their full length, or withdrawn so as to be 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 into 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, Vv^hich 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 iii 

 the lining membrane ; the segments behind this are 

 now drawn up close, and extend their bristles ; these 



