WHEEL-ANIMALCULES. 



45 



aquaticus is the thread-worm, common in many insects 

 and in pools. They are the so-called horse-hairs that do 

 not turn to snakes. 



Class III. WHEEL- ANIMALCULES (Rotifers). 



The Rotifers (Fig. 44) are microscopic worms, some 

 being only T f^ of an inch in length, having a membranous 

 covering. Upon the anterior por- 

 tion they have one or two disks, 

 surrounded by cilia (Fig. 44, A), 

 whose constant motion creates 

 the optical illusion of wheels re- 

 volving. They abound in the 

 ocean, and in standing fresh wa- 

 ter, and are so tenacious of life 

 that they recover after having 

 been dried for years, their sud- 

 den appearance after continued 

 droughts being attributed to what 

 is called spontaneous generation. 

 Under the microscope they as- 

 sume remarkable shapes : now 

 oval, caused by the upper and 

 lower segments shutting togeth- 

 er ; then leech-like, moving along 

 seemingly by suckers, or darting 

 off swiftly, propelled by cilia. 

 They are oviparous, and Ehren- 

 bcrg found that one species pro- 

 duced 16,000,000 young in twelve 

 days. 



FIG. 44. A Rotifer, highly 

 magnified (Hydatina sen- 

 id). A, cilia ; #, anus ; 

 , contractile vesicle ; c, 

 water-vessels ; e, ovary : 

 y, ganglion. 



Class IV. Moss ANIMALS (Poly sod). 



These animals form a moss or coral-like growth, com- 

 posed of cells (Fig. 45), each of which contains a minute, 



