200 AQUATIC MICROSCOPY FOR BEGINNERS. 



with several finger-like prominences surroundingit. In 

 such worms these parts are ciliated on the inner side, 

 the currents thus produced being supposed to bring 

 at least a portion of the oxygen needed for respiration. 



The alimentary canal extends through the center of 

 the entire body, and ih usually crowded with the 

 brownish remains of undigested food. 



The whole cavity of the body outside of the aliment- 

 ary canal is filled with a colorless fluid visible only by 

 means of the movements of the corpuscles often to be 

 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 and contained in two distinct longitudinal vessels, 

 one extending lengthwise above, the other lengthwise 

 below the intestine. These vessels unite at both ends 

 of the body, so as to form a long, closed tube, with 

 branches springing from the front part, or from the 

 upper or dorsal tube as it passes through each seg- 

 ment, where they, then appear as pulsating loops. 

 Usually the blood is impelled by the regular pulsa- 

 tions of the dorsal vessel, a wave-like contraction pas- 

 sing along and driving the fluid before it. In two 

 genera (Tiibifex and Ocnerddrilus) there are little pul- 

 sating hearts attached to the dorsal vessel in the 

 neighborhood of the frontal border. 



Reproduction is by eggs or by tranverse fission, the 

 latter being most frequently observed. 



Most of these worms live upon animal food, s.eem- 

 ing to prefer Rhizopods and Rotifers to almost any- 

 thing else; only a few are vegetarians. 



