HOOKS OF THE SKltPVLA. 135 



do is to withdraw the hooks, straighten the body, and down ll 

 falls by its own weight, 



IN the nautical branch of this subject I have already treated 

 of the curious pushing-poles by means of which the Serpula 

 protrudes itself from its tube. As all must have noticed who 

 have seen these creatures alive, the Serpula protrudes itself very 

 slowly, but flies back into its tube with such velocity that the 

 eye can scarcely follow its movements. Its difference of motion 

 shows that there must be a difference in the means by which 

 these movements are produced. 



Referring to the illustration on page 45, the reader will see 

 that the instruments with which the Serpula propels itself are 

 used just after the fashion of punt-poles, and cannot act with 

 any great swiftness. When, however, the creature wishes to 

 withdraw itself, it employs a curious apparatus, consisting of 

 many rows of little hooks. The points of these hooks readily 

 catch against the lining of the tube, and by their aid the worm 

 jerks itself back with wonderful celerity. 



Three rows of these hooks are shown next to the Tiger-beetle 

 larva. 



The structure of these remarkable organs is elaborately de- 

 scribed by Mr. Gosse in his " Evenings with the Microscope:" 



"If you look again at this Serpula recently extracted, you 

 will 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 exceedingly delicate membrane, and, 

 on placing it under a high power (say six hundred diameters), 

 you will be astonished at the elaborate provision here made for 

 prehension. 



" This yellow line, which cannot be appreciated by the un- 

 assisted eye, is a muscular ribbon, over which stand edgewise a 

 multitude of what I will call combs, or rather subtriangular 

 plates. These have a wide base, and the apex of the triangle 

 ifi curved over into an abrupt hook, and then this cut into a 

 number (from four to six) of sharp and long teeth. 



" The plates stand side by side, parallel to each other, along 

 the whole length of the ribbon, and there are muscular fibres 

 seen affixed to the basal side of each plate, which doubtless 

 give it independent motion. 



