THE BEISTLE-WORMS. O6 



purposes are the acquisition of a practical knowledge of the 

 structure of worms, we shall begin with the bristle-worms, 

 or Chsetopoda. They are more highly differentiated than 

 the ribbon-worms, are often of considerable size, and are 

 easy to examine and dissect. 



The Chsetopoda (or " bristle-feet ") include two main sets of 

 worms the marine forms (the Polychaetes), worms usually 

 with many bristles, arranged on lateral outgrowths of the 

 body (the parapodia or feet), and the Oligochsetes, worms 

 like the common earthworm, living in earth or in fresh 

 water, and having only few bristles. It is the marine 

 worms only with which we are concerned here. 



The first step is, of course, to find the worms, but this is 

 considerably easier than in the case of the historic hare. 

 There is no shore so barren and desolate that it does not at 

 some point or other show traces of the bristle-worms. On 

 the mud-flats at the mouths of the rivers, on the smooth 

 sandy shore at the edges of the rocks, or in the sandy bays 

 in the middle of the rocks, one finds in abundance the 

 "castings" of the common lob-worm. The dark seaweed 

 thrown up by the breakers nearly always bears upon its 

 fronds the little coiled dead-white tubes formed by the tiny 

 Spirorbis. Among the debris which accumulates at tide- 

 mark, a careful scrutiny will almost always reveal the neatly 

 made tubes of Terebella decorated with particles of shell 

 and stone, and encircled at the tip by a fringe of stiff sandy 

 threads. The shore rocks are often in places covered with 

 masses of the sandy tubes of Sabellaria, which look them- 

 selves like an outcrop of porous rock. We might, indeed, 

 continue the list almost indefinitely, but let us first choose a 

 typical form for closer study. 



In turning over stones on the rocks between tide-marks, 

 especially in slightly muddy pools, you are almost certain 

 sooner or later to dislodge the worm for which we are 

 seeking (see Fig. 28). When disturbed by the removal of 

 the stone under which it has been lurking in an ill-defined 

 burrow, it swims away with a peculiar wriggling motion. The 

 colour is brown or greenish, and there is usually a faint but 

 distinct metallic sheen. The length may be as much as six 

 inches, but in forms from shallow water it is likely to be 

 considerably less. The upper surface is arched, the lower 



