THE WHEEL- ANIMALCULE. 69 



work ; and the cells of the different tissues, like the indi- 

 vidual members of society, by giving themselves entirely 

 to one kind of work, make themselves specially fit for this 

 work, and become almost completely unfitted for anything 

 else. 



This is what is meant by the physiological division of 

 labor. 



Eead the descriptions of Amoeba in Brooks' " Handbook 

 of Invertebrate Zoology," and in "Practical Biology," by 

 Huxley and Martin ; the characters of Protozoa, in Pack- 

 ard's " Zoology " ; the lecture " On a Piece of Chalk," in 

 Huxley's " Lay Sermons, Addresses, and Reviews " ; the 

 Introduction to Foster's " Physiology." 



THE WHEEL-ANIMALCULE (ROTIFER). 



Rotifers are often found in the water of an aquarium 

 where clams and crayfishes have been kept ; pick out 

 clusters of plant growth, found in the rubbish and sedi- 

 ment in the aquarium, or on the shells of clams ; with a 

 lens look at the walls of the aquarium for small, white, 

 worm-like 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 

 presence of a distinct digestive tube, a distinct nervous 



