72 TEXTBOOK OF ZOOLOGY 



the most common living genus. Epistylis and Carchesium are well- 

 known colonial genera. Vorticella and Carchesium have contractile 

 stalks while Epistylis is attached by noncontractile branching stalks. 



The second subclass, Suctoria or TentacuUfera, as it is sometimes 

 called, includes animals that are not ciliated, except during a free- 

 swimming stage which may occur following division or encystment. 

 These are attached forms with protoplasmic projections which are 

 used in the capture of food. Most of them are marine, but Podophrya 

 is an example of a fresh-water genus. 



4. Class Sporozoa (sporozo'a, seed animal). — These protozoans 

 in their early stages are often amoeboid, but in the completed life 

 history locomotor structures are wanting. During the life cycle there 

 is a spore stage. The animals of this class are entirely parasitic 



^^(^^''f^^T^ Ep i th e Hum 

 ; i<m^l . :,; hfH-Early sta§e 



.Intermediate sta^e 



-natarc 5ta^e 



Fig. 29. — Gregarina attached to an epithelial cell of host's Intestine. Other 

 stages of its development are shown within adjacent cells. 



and they are usually transmitted to other animals in the spore 

 stage. They often pass from one host in its feces and enter another 

 in contaminated food or drink ; or they are drawn from one host by 

 a blood-sucking animal and transmitted to the blood of ajiother. 

 All Sporozoa reproduce by sporulation in which asexual, multiple 

 fission is followed by gamete formation, and the gametes fuse to 

 form a zygote. The spores are produced by the parent animal 

 dividing into fragments while it is encysted. These little cysts, 

 which are secreted by the protoplasm of the animal, are protective 

 and enable them to withstand adverse conditions. The cyst is dis- 

 solved upon entrance to a host and liberates the organisms. 



This class of Protozoa is among the most widely distributed of 

 the animal parasites, and their life cycles are often quite compli- 



