238 



SUEKINGDOM ARTICULATA. 



Fig. U07. 



The Hinidinidae (Leeches) are aquatic without bristles 

 or gills. They are used in medicine for bleeding, France 

 alone using a hundred million annually. A saw-like move- 

 ment of their tri-radiate jaws readily cuts the skin, and with 

 the aid of the vacuum produced by the sucker, causes a 

 copious flow of blood. So slow is its 

 digestion that a single meal will answer 

 for a year. Over thirty species are 

 known to inhabit the fresh waters of 

 North America. (Fig. 406.) 



-ORDER ROTATORIA. 



. The Rotifers (wheelbearers) are 

 aquatic and microscopic, seldom ex- 

 ceeding one thirty-sixth of an inch in 

 length. Anteriorly they have one or 

 two discs surrounded by cilia, whose 

 rapid motion produces the optical illu- 

 sion of revolving wheels. They are ovi- 

 parous, and, according to Ehrenberg, 

 one species multiplied in twelve days, 

 Wheel Aiimaicuie, greatly to sixteen millions, as " determined 

 by actual experiment," The upper 

 and lower segments shut together like a telescope, and the 

 animal often assumes a spherical shape. By some the Eoti- 

 fers are considered Crustacea. 



Rotifer vulgaris. 

 Animalcule, 

 magnified. 



Echinorhynchus gigas, Spine-headed Worm. 

 ORDER ACANTHOCEPHALI. 



Acanthocephalidae. The Acanthocephali (spine-head- 

 ed) are represented by Echinorfiynchus, a parasite found in 



