SOME AQUATIC WORMS, ETC. 185 



therefore called podal or foot spines. They can be 

 protruded from the body, or partially withdrawn into 

 it, at the animal's will. The long bristles are used to 

 assist in swimming. In some of these worms both 

 the bristles and the podal spines are present, in others 

 one or the other set of organs may be absent. 



The spines, which with but few exceptions are pres- 

 ent, are each gracefully curved like a long italic S, 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 

 (Fig. 141). The body or shaft of the spine has, at some 

 point of its length, a globular enlargement or a shoul- 

 der, below which the spine is often much narrowed. 

 These organs are used by being protruded and forced 

 against the surface over which the worm is travelling. 

 They are arranged in a row on each side of the ventral 

 surface, each row being composed of many clusters, and 

 each cluster of from two to ten podal spines. The worm 

 can protrude several clusters at once, or two on the op- 

 posite sides of the same segment or body-ring, but it 

 seems unable to extend them irregularly. 



The bristles are very flexible, and are arranged in two 

 rows on the sides, near the upper surface, one row on 

 each side. They are usually much longer than the 

 width of the body, and may be so arranged that there 

 are several or only one on each lateral margin of 

 the segment. They are sometimes accompanied by a 

 straight spine much shorter than the bristle, and pro- 



