PHYLUM PROTOZOA 43 



their diversity of beautiful forms and pleasing colors. Though 

 most have no direct economic importance, Plasmodiophora 

 brassicae is parasitic upon cabbage and related plants. 



Class Sporozoa 



The Sporozoa are Protozoa without locomotor structures in the 

 adult stage. While representing great diversity of structure, 

 habitats, and life cycle, they all agree in having the parasitic 

 habit firmly established and in reproducing by spores which are 

 usually enclosed in a firm shell. In some instances, especially 

 where alternation of hosts is introduced, the spore shell may be 

 wanting and in still other instances more than one spore may be 

 included within a single shell. Alternation of generations is 

 widely distributed in this group. Sporozoa are usually intra- 

 cellular parasites, at least during the early stages in develop- 

 ment. In most instances the spores give characters more readily 

 available for identification than do the vegetative stages. Nutri- 

 tion is exclusively by absorption through the body surface. 



Representatives of many groups ranging from Protozoa to 

 mammals serve as hosts to sporozoan parasites. Pathological 

 conditions in the host frequently result from infection by these 

 parasites. The organisms which cause malaria (Fig. 29) are by 

 far the most important members of this class. 



Subclass Telosporidia 



In members of this subclass spore formation occurs only at the 

 end of the vegetative period and sporulation results in the 

 destruction of the vegetative individuals. Typically, the adult 

 stage has a single nucleus but in some a multinuclear stage is 

 found. Infection of a new host is through a stage called a 

 sporozoite which is usually an intracellular parasite. The sporo- 

 zoite either develops directly into a sexual individual or first 

 passes through an asexual multiplicative cycle. Following ferti- 

 lization, spores are formed which give rise to the sporozoites. 



The gregarines, the Coccidia, and the malarial organisms are 

 the chief representatives of this subclass. Of these the gregarines 

 five in invertebrates exclusively, being found chiefly in arthropods 

 and annelids where they dwell as parasites in the lumen of the 

 digestive tract and of glands opening into it. Typically the 

 gregarine body is divided into an anterior protomerite and a 

 posterior deutomerite separated by an ectoplasmic septum. Of 



