PROTOZOA 



Fig. 6 



A stalked 

 infusorian (C'o- 

 thurnia socialis). 



Fig. 5. — A bell-animalcule ( Vorticella 

 microstoma). A, extended; B, re- 

 tracted; C, encysted; c, cilia; /, 

 balls of food; n, nucleus; p, pedun- 

 cle ; r, contractile vacuole. 



circle of cilia around the farther end, and sets them in such a rapid 

 motion that they resemble the spokes of the wheel of a rapidly pass- 

 ing carriage. Closer observation 

 shows that this motion creates 

 currents in the water, and that 

 these currents have a uniform 

 direction, and always pass down 

 into a cavity hollowed out of the 

 bell. The currents bring with 

 them a quantity of minute par- 

 ticles, and these soon form a 

 ball of considerable size at the 

 bottom of the cavity. This ball 

 then passes into the protoplasm 

 of the body, and there under- 

 goes digestion, like the food of 

 the Amoeba. Another feature which also de- 

 serves mention is the contractile vacuole. This 

 is a space in the body of the Vorticella, which 

 gradually enlarges, and then suddenly collapses, to again repeat the 

 operation. It is supposed to be respiratory and excretory in its functions. 



Others of the Infusoria are free, moving or 

 swimming about at will. Some live a parasitic 

 life in other animals, and one has even been 

 claimed to be the cause of hay-fever. Among 

 the flagellate forms, those with a delicate collar 

 around the flagellum are most interesting, since 

 they recall the feeding-cells of 

 the sponges, to be described far- 

 ther on. 



A third great division of Pro- 

 tozoa, the Gregarinida, need to 

 be but mentioned. All are para- 

 sitic in the digestive tract or 

 body cavity of other animals. 



In all of the remaining groups 

 of animals the body consists of 



Fig. 7. — A ciliated Protozoan 

 (Stylonichia mytilus) greatly 

 enlarged. 



a large number of cells. In the 

 Protozoa, as we have seen, the 

 single cell performs all the func- 



Fig. 8. — A collared 

 flagellate infuso- 

 rian {Salpingceca 

 acuminata). 



