224 



THE ANIMAL KINGDOM 



Figure 12.3. Dorsal view of the rotifer, Rotaria, showing external structures and 

 many of the muscle strands in the body wall. Similar structures occur in Philodina. 



empties every few minutes, suggesting that the primary function of the 

 rotifer excretory system is water balance. A pair of ovaries lateral to 

 the stomach also open into the bladder by paired oviducts. The bladder 

 opens dorsally at the base of the foot. 



Philodiyia may detach itself and swim away. The action of the 

 wheel-organ, which pulls water toward the animal in feeding, is equally 

 suitable for locomotion. Rotifers often swim off in this fashion when they 

 are disturbed. 



When a rotifer creeps on the bottom or on vegetation, its entire 

 wheel-organ is retracted into the body by its retractor muscles (Fig. 

 12.3). The rostrum is everted and forms a new anterior end to the body, 

 dorsal to the wheel-organ. At its tip are cilia, spines and plates by which 

 it can attach. Like a leech or bloodsucker (Chapter 15) with its anterior 

 and posterior suckers, Philodina creeps by alternately attaching rostrum 

 and foot. When it finds a place suitable for feeding, the rostrum is re- 

 tracted by means of the rostral retractor muscles as the wheel-organ is 

 everted. In a sense Philodina has two anterior ends which it can use 

 alternately. The retractor muscles are part of the body wall musculature, 

 which also includes a number of circular and longitudinal strands (Fig. 

 12.3). The remainder of the body wall is made of a simple ectodermal 

 epithelium covered by the cuticle. In the creeping rotifers this cuticle 

 is segmented to facilitate movement. 



The nervous system of rotifers, like that of the flatworms, includes 

 a bilobed brain dorsal to the pharynx, and several pairs of nerves, of 

 which the ventro-lateral pair are the largest. Additional nerve cell 

 ganglia are located on the pharynx, bladder and foot. 



Sense organs include bristles for touch and chemoreception on the 

 body, especially around the wheel-organ and rostrum. Most rotifers have 



