182 ITS ARRAY OF HOOKS 



being now retracted, they yield to the movement and 

 are pushed forward, while the others are held firm by 

 the resistance of their holding bristles ; thus gradu- 

 ally the foreparts of the animal are exposed. 



But this gradual process would ill suit the necessity 

 of a creature so sensitive to alarm, when it wishes to 

 retreat. We have already seen how, with the fleetness 

 of a thought, its beautiful crown of scarlet plumes 

 disappears within its stony fastness ; let us now look 

 at the apparatus which effects this movement. 



If we look at a Serpula recently dead, — which we 

 may readily do, since it is the habit of most tubicolous 

 Annelida to come out to die, — we shall find, with a 

 lens, a pale yellow line running along the upper surface 

 of each foot, transversely to the length of the body. 

 This is the border of an exessively delicate membrane, 

 and on placing it under a high power (say 300 dia- 

 meters) we are astonished at the elaborate provision here 

 made for prehension. This yellow line, which cannot be 

 appreciated by the unassisted eye, is a muscular ribbon, 

 on which stand up edgewise a multitude of what I 

 will call combs, or rather sub-triangular plates. The 

 edge of each plate is cut very regularly into six teeth, 

 which curve in one direction, and one other curved so 

 as to face these. The combs stand side by side, 

 parallel to each other, along the whole length of the 

 ribbon, and there are muscular fibres seen affixed to 

 the smaller end of every plate, which doubtless give it 

 independent motion. I counted 136 plates on one 

 ribbon ; there are two ribbons on each thoracic seg- 

 ment, and there are seven such segments : — hence we 

 may compute the total number of prehensile comb- like 



