CHAPTER 5 



OJI 



Phylum Protozoa. 



OneCelled 



Parasites 



HE class Sporozoa lives on or within other 

 animals from which it derives nutrition, and 

 therefore these animals are classified as para- 

 sites. They are not as well known as other 

 types of protozoans, but they may be found 

 in animals ranging in complexity from sim- 

 ple invertebrates to man. The life cycles of 

 many sporozoans involve different species 

 of hosts. A host is any plant or animal on 

 or within which a parasite lives and from 

 which it obtains its nourishment. The var- 

 ious stages of development of the sporozoans 

 are very interesting, and the methods by 

 which they are transmitted from one host 

 to another are quite remarkable. They may 

 also cause the death of their host, including 

 man, and are therefore very important to 

 our welfare and economy. 



The life cycle is usually complicated, in- 

 volving the production of resistant stages 

 which in some cases are called spores. The 

 spore may be a spindle-shaped case contain- 

 ing sporozoites (Fig. 23). Spores serve as an 

 infective stage in the life cycle. They often 

 pass out of one host in the feces and enter 

 another host in contaminated food or drink; 

 or they may be sucked out of one host by a 

 bloodsucking animal, such as an insect, and 

 inoculated into another animal by this in- 

 termediate host. 



MONOCYSTIS LUMBRICI-A 



SPOROZOAN PARASITE 



OF EARTHWORMS 



50 



Monocystis lumbrici illustrates many of 

 the characteristics of the Sporozoa (Fig. 23). 

 It is a parasite almost invariably found in 

 the seminal vesicles of the common earth- 

 worm. The stages that are usually present 

 are (1) the trophozoite, (2) cysts contain- 

 ing two individuals, or gametes and spores 

 in various phases of development, and (3) 

 isolated spores. 



Monocystis is easily obtained for study. 



