SOME AQUATIC WORMS, ETC. 199 



These organs are used by being protruded and forced 

 against the surface over which the worm is traveling. 

 They are arranged in a row on each side of the vent- 

 ral surface, eachrow being composed of many clusters, 

 and each cluster of from two to ten spines. The 

 worm can protrude several clusters at once, or two on 

 the opposite sides of the same segment or body-ring, 

 but it seems unable to extend them any more irregu- 

 larly. 



The bristles are exceedingly flexible, and are ar- 

 ranged in two rows on the sides near the upper surface, 

 one series on each side. They are usually much 

 longer than the width of the body, and may be so ar- 

 ranged with several or with only one on each lateral 

 margin of the segment. They are sometimes accompa- 

 nied by a straight spine much shorter than the bristle, 

 and projecting beside it. The free ends of these rudi- 

 mentary spines are occasionally finely forked. The 

 bristles are absent in some genera. 



The aquatic worms are usually visible to the naked 

 eye as fine whitish or yellowish threads, sometimes an 

 inch or more in length when extended. They are 

 found abundantly among aquatic plants, and in the 

 mud of shallow ponds. When allowed to remain in 

 the collecting-bottle they will often make their way to 

 the lighted side, where some will form sheaths or pro- 

 tective cases from bits of Lemna or various floating 

 ■fragments or particles. 



The mouth may be close to the front end, or at some 



distance back, since in a few worms the front border 



is extended in a long flexible snout. The posterior 



^^ border of the body is rounded in many forms, while in 



others it is expanded into a broad, funnel-like region, 



