CHAPTER XIX. 

 TROCHELMINTHES. 



THE WHEEL ANIMALCULE (ROTIFER). 



ROTIFERS are often found in the water of an aquarium where 

 clams and crayfish have been kept ; pick out clusters of plant 

 growth, found in the rubbish and sediment in the aquarium, or 

 on the shells of clams ; with a lens look at the walls of the 

 aquarium for small, white, wormlike forms. 



The body of the wheel animalcule is tapering, ending in a two- 

 forked foot. At the larger end, when expanded, are two circular 

 disks, fringed with cilia ; the disks are retractile, as in Vorticella. 

 Between the disks is the mouth ; this opens into the pharynx, 

 lined with teeth ; back of the pharynx are the stomach and 

 intestine. 



Rotifers are classed with the worms ; though small, the pres- 

 ence of a distinct digestive tube, a distinct nervous system, and 

 organs of sight and hearing, show the Rotifer to be much more 

 highly developed than the protozoans. 



Rotifers have been dried and kept for years, and yet when put 

 into water they revived. 



Study carefully : 



1. The mode of locomotion. 



2. The action of the disks and cilia. 



3. The motions of the pharynx. 



4. The contraction and expansion of the body as a whole. 

 Make drawings showing the body both in the expanded and in 



the contracted state. 



Read the " General Characters of Rotifers " in Packard's 

 Zoology; "Rotifera" in Claus and Sedgwick's Text-book 

 of Zoology ; " Trochelminthes " in Parker and Haswell's Text- 

 book of Zoology. 



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