186 MICROSCOPY FOR BEGINNERS. 



jecting beside it. The free ends of these rudimentary 

 spines are occasionally finely forked. The bristles are 

 absent in some genera. 



The worms are usually visible to the naked eye as 

 very 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 col- 

 lecting-bottle they will often make their way to the 

 lighted side, where some will form sheaths or protect- 

 ive cases from various floating fragments or particles. 



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

 distance back, since in a few worms the front border is 

 extended as a long, flexible snout. The posterior border 

 is rounded in many forms, while in others it is expanded 

 into a broad, funnel-like region, with several finger-like 

 prominences surrounding it. In such worms these 

 parts are ciliated on the inside, the currents thus pro- 

 duced being supposed to bring at least a portion of the 

 oxygen needed for respiration. The alimentary canal 

 extends through the centre of the entire body, and is 

 usually crowded with the brownish remains of undi- 

 gested food. The whole cavity of the body outside of 

 the alimentary canal is filled with a colorless fluid visi- 

 ble only by means of the movements of the corpuscles 

 seen floating to and fro as the worm moves under the 

 cover-glass. 



The beginner must not mistake this fluid for the 

 blood, which in many of the bristle-bearing forms is red 



