338 



SUBKINGDOM AETICDLATA. 



The Hirudinidae (Leeches) are aquatic without bristles 

 or gills. They are used iu medicine for bleeding, Prance 

 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 



I 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 

 I 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 procfuces the optical illu- 

 sion of revolving wheels. They are ovi- 

 parous, and, according to Ehrenberg, 

 one species multiplied in twelve days, 

 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 Koti- 

 fers are considered Crustacea. 



nn. im 



Rotifer vulgaris. 



Wheel Animalcule, greatly 



magnified. 



Ecliinorkynchus gtgoi:, Spme-neaued Wuiin. 



ORDER ACANTHOCEPHALI. 



Acanthocephalidse. — The Acanthocephali (spine-head- 

 ed) are represented by Ecliinorhynchus, a parasite found in 



