222 ^"^ ANIMAL KINGDOM 



V. Gordiacea. Long, slender, cylindrical worms widi a reduced 

 digestive system and no excretory system. Parasitic as juveniles. 

 VI. AcanJhocephala. Parasitic worms that (like the tapeworms) lack 

 a digestive system. They have a retractile spiny head. 



82. Class Rotifera 



Rotifers, which are about the size of paramecia, are among the most 

 abundant microorganisms in ponds, lakes and streams. Some fifteen 

 hundred species are known. A few of these live in moss or wet sand, 

 others live in the oceans, but the majority live in fresh water. Some 

 rotifers float in the water, others are attached to the bottom or to other 

 animals, and still others creep about with leechlike movements. Most 

 of the familiar rotifers, common in temporary ponds and puddles, are 

 of the creeping variety. 



A characteristic structure of the rotifers is the wheel-organ, a 

 circlet of cilia extending around the front end of the head from the 

 antero-ventral mouth (Fig. 12.2). It may be a simple circle, or it may be 

 elaborated by outfoldings from the body. A common plan is that of a 

 double circle (Fig. 12.2). When observed under the microscope the 

 wheel-organ appears to rotate, an illusion so convincing that Leeuwen- 

 hoek believed rotifers possessed wheels. The illusion is the result of a 

 coordinated rhythm of the ciliary beating. The cilia beat in waves, 

 which pass circularly around the rim. At any given moment some cilia 

 are relaxed while the adjacent ones are bending, producing a momentary 

 aggregation of cilia. It is these aggregations of cilia moving with the 

 waves around the circle that are seen, and not the motion of the indi- 

 vidual cilia. 



Most rotifers are transparent and the internal jaws are easily seen, 

 especially since they are usually in motion. In spite of their small size 

 the jaws are elaborate, being composed of seven pieces of varying shape. 

 In different species, they may be used for grinding, biting or piercing. 

 When used for biting or piercing the jaws are everted through the 

 mouth. 



The body, which is clothed in a thin cuticle, usually ends poste- 

 riorly in a foot (Fig. 12.2). The foot is equipped with pedal glands 

 that secrete adhesive mucus, by which the rotifer can attach to objects 

 temporarily or permanently. The foot is missing in many of the plank- 

 tonic species. 



83. Philodina 



The genus Philodina includes a number of common species of 

 creeping rotifers, of which P. roseola (Fig. 12.2) is representative. Its 

 wheel-organ is divided into two whorls, with the funnel-shaped mouth 

 located midventrally between them. Philodina is usually attached by its 

 foot and creates water currents with the wheel-organ that bring minute 

 food particles (algae, bacteria, etc.) to the mouth. 



The mouth leads to a muscular pharynx containing hard cuticular 



