KOTIFERS. 201 



eyes. They are often to be noticed near the front of 

 the body in young individuals, but in the old they are 

 as often absent. Their number and position are some- 

 times used as characters by which the genera and spe- 

 cies are classified, but, since they disappear with age, 

 they cannot be of much value for this purpose, certainly 

 not to the beginner. 



The body is inclined to be cylindrical, yet there are 

 some resembling flat disks and oblong figures. Neither 

 are they all free-swimming. Some are permanently ad- 

 herent to the leaflets of aquatic plants or other sub- 

 merged objects, but these generally form a protective 

 sheath about themselves, into which they retire when 

 frightened or disturbed, in a manner similar to that of 

 some Infusoria ; and, as in the Infusorial loricse, the 

 sheaths may be formed of a stiff membrane, or of the 

 softest and most gelatinous material, or they may be 

 built of particles of dirt or rejected food fragments. In 

 all instances the sheaths are the work of the Rotifers 

 inhabiting them, and none of the Sheath-building Roti- 

 fers are free-swimmers.* Most of the free swimmers, 

 however, may become temporarily adherent by means of 

 their foot and toes. The body of these free-swimming 

 forms may be soft and flexible, and without any greater 

 protection than is afforded by the skin, or it may be en- 

 closed within a hard, shell-like coating called a carapace. 

 The bodies of all the sheath-building Rotifers are with- 



* Since the above was written three have been discovered in Eng- 

 land. But this need not trouble the beginner. 



