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 gradually 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 diameters) 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 segment, and there are seven such segments:—hence we may compute the total number of prehensile comb-like
