198 AQUATIC MICROSCOPY FOR BEGINNERS. 



gudlula aceti) ; and the paste-worm (A. gliHims) belongs 

 to the same genus. Some naturalists regard these as 

 the same species. 



VI. OLIGOCH^TA. 



The fresh-water, bristle-bearing worms whose bodies 

 are never ciliated, show more or less distinctly that 

 they are formed of segments or rings. 



Each segment usually has on' both sides near the 

 back one or more long, fine, hair-like bristles extend- 

 ing outward into the water, and together forming a 

 series along each side of the body. On the lower 

 surface are two or more rows of stouter, inflexible, 

 gracefully curved spines, the rows being formed of 

 clusters of two or more, the free end of each spine be- 

 ing usually divided by a deep notch, so that it appears 

 like a double hook, the parts being unequal in size and 

 in degree of curvature. These spines are used to 

 assist in the worm's movements and are therefore called 

 podal- spines or foot-spines. They can be protruded 

 from the body, or partly withdrawn into it, at the ani- 

 njal's will. The long bristles are used as an assistance 

 in swimming. On some of these worms both bristles 

 and podal spines are present; in others one or the 

 other set of organs may be absent. 



The podal spines, which with but few exceptions are 

 present, are each gracefully curved like a long italic 5, 

 their shape resembling the line which artists have 

 called the 'line of -beauty.' The free end, or the one 

 projecting into the water, is forked in a way already 

 'described, and shown in Fig. 154. The body or shaft 

 has, at some pointof its length, a globular enlargement or 

 a shoulder, below which the spine is often much narrowed. 



