SOME AQUATIC WORMS, ETC. 187 



and contained in two distinct vessels, one extending 

 lengthwise above, the other 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 segment, where they then appear as pul- 

 sating loops. Usually the blood is impelled by the ir- 

 regular pulsations of the dorsal vessel, a wave-like con- 

 traction passing along and driving the blood before it. 

 In two genera (Tubifex and Ocnerodrilus) there are 

 little pulsating hearts attached to the dorsal vessel in 

 the neighborhood of the frontal border. 



Eeproduction is by eggs or by tranverse fission, the 

 latter being most frequently observed. 



Most of these worms live upon animal food, seeming 

 to prefer Khizopods and Rotifers to almost anything 

 else ; a few are vegetarians. 



Key to Genera of Microscopic, chiefly Fresh-water, Worms 

 ( Oligochceta). 



1. Body with both bristles and podal spines (a). 



2. Body with podal spines only (J). 



3. Body with bristles only (/"). 



a. Anterior extremity with a flexible, finger-like pro- 

 longation. Pristina, 1. 



a. Anterior extremity without a finger-like prolon- 



gation (d). 



b. Podal spines forked ; worms aquatic (c). 



I. Podal spines not forked ; worms aquatic (g). 

 9* 



