540 EVOLUTION OF ANIMALS Part V 



even of man. In rotifers, there is a definite number of cells according to the 

 species, and so few that they can be easily counted. Adults are peculiar in that 

 the whole body may be composed of incomplete cells (syncytia). In Hydatina 

 senta, there are about 1000 nuclei in every adult and in embryos of the same 

 species about 1000 complete cells are formed. Later, the cell membranes dis- 

 appear but the number and locations of the nuclei remain definite and in the 

 same relative position as in the embryo. 



Other Functions. The corona helps in locomotion, respiration, and get- 

 ting food. Whether a rotifer is swimming or creeping, the strong backward 

 strokes of the cilia on the corona drive it forward. Rotifers are delicately 

 responsive to their surroundings and the activity of the cilia is quickened or 

 slowed accordingly. Their beat continually bathes the animal with fresh water, 

 provides oxygen and food, at the same time carrying away the carbon dioxide. 

 Particles floating in the water are brought to the mouth at the center of the 

 whirlpool (Fig. 27.7). The rotifer needs only to open its mouth. 



Whatever the food may be, whole cells or fragments, plants or animals, it 

 is whirled into the grinding mastax (Fig. 27.8). It then passes through the 

 esophagus to the stomach where chemical digestion is carried on by the secre- 

 tion from two large gastric glands. Digested food is absorbed through the walls 

 of the stomach and intestine and on into other regions of the body. The un- 

 digested remains pass out through the anal opening. It is to be remembered 

 that rotifers swallow great numbers of diatoms, all of them encased in silicious 

 shells considerably harder than glass. In some species, the digestive canal does 



Fig. 27.7. Currents of water produced by the cilia of the corona of a rotifer, 

 Proales. The fine dots represent particles drawn in a vortex toward the mouth as 

 the animal moves toward the right. The cilia strike backward more strongly than 

 forward and thus produce currents of water that pass backward from in front 

 of the animal to its mouth and over the surface of its body. Thus, they bring 

 food and a continual supply of oxygen to the surface of the body. (Courtesy, 

 Ward and Whipple: Fresh Water Biology. New York, John Wiley and Sons, 1918.) 



