n"he Wheel Animalcules 



DR. E. BADE 



Among the minute animals which peo- 

 ple the water, the rotifers or "wheel 

 bearers," Rotatoria, form a very inter- 

 esting class. Only a few of them reach 

 even three millimeters in length, and they 

 are giants of the race ; usually they vary 

 between one-twentieth and one-tenth mm. 

 in length (a millimeter is approximately 

 one twenty-fifth of an inch). These lit- 

 tle fellows put dramatic movements into 

 the life of the pond. Some sport around 

 in the clear water, "Knights of the Lists," 

 bearing mighty lances, as do others 

 thorns. Others are sessile on plants, on 

 the lower surface of lily pads, for in- 

 stance. A few live in the sea ; some, in 

 the intestines of worms and molluscs, lead 

 parasitic lives, but the species most nu- 

 merous are those that prefer the quiet 

 backwaters of streams, the bottoms of 

 plant-grown ponds or the puddles in 

 swamps. One family, the Bdelloidae, oc- 

 curs in the moss of house-roofs and in 

 the lichen-growths of tree-trunks and 

 rocks. 



The older naturalists called them wheel 

 animals, and this popular name has per- 

 sisted. The anterior end of the body car- 

 ries a retractile ciliated apparatus, the so- 

 called "wheel organ," which varies con- 

 siderably in appearance in the different 

 species. This organ, thickly beset with 

 cilia, has a twofold purpose, serving for 

 locomotion and also creating a current 

 in the water whereby edible substances 

 are brought to the ever hungry maw. The 

 "wheel" appears circular for one mo- 

 ment, escalloped the next, then frilled, 

 lobed, even branched or armlike. Through 

 a compound microscope the ceaseless play 



of the cilia gives the impression of the 

 spokes of a revolving wheel. This ap- 

 pearance is so deceptive that the first ob- 

 servers assumed it as a fact that the ani- 

 mals carried a wheel ; hence the name. 



By adding a little cocaine or quince- 

 gum to the water in which the animals 

 are being examined microscopically the 



Brachionus urceolaris, Melicerta ringens 

 and Stephanoceros eichhorni 



play of the cilia slackens, and it can then 

 be seen that the apparent wheel in motion 

 is simply very minute hairs (the cilia), 

 which rhythmically beat the water. But 

 the little wheel organ is not the sole point 

 of interest with these fellows. Even now 

 their exact position in the zoological sys- 

 tem is not quite clear. 



