THE PHYLUM PROTOZOA \Q^ 



cirri with which the animal scrambles over surfaces. A third order is 

 tiie Peritricha, in which the cilia are limited to a counter-clockwise 

 spiral leading to the mouth. Tlie cilia are usually not fused to form 

 membranelles. Most of the peritrichs are attached by stalks, and many 

 species are colonial (Fig. 8.13). 



51. Class Suctoria 



The suctorians (Fig. 8.1) are an offshoot of the ciliates which retain 

 both macronucleus and micronucleus. The sedentary adults have no 

 cilia but usually have stalks. The body bears a group of tenl'acles that 

 are used for feeding. When prey such as other protozoa happen to strike 

 the end of a tentacle, they adhere and are paralyzed by a toxic secre- 

 tion. The contents of the prey are then sucked through canals in the 

 tentacles and drawn into the bodv of the suctorian. 



Although adult suctorians lack cilia they do possess the basal bodies 

 of cilia. During asexual reproduction the suctorian forms a bud in 

 which the basal bodies multiply, become arranged in rows, and develop 

 cilia similar to those of a holotrich. After nuclear division the bud 

 separates and swims away. It later attaches to the bottom, the cilia 

 disappear, and tentacles develop. 



In view of the obvious ciliate affinities the suctorians are often 

 considered to be an order in the class Ciliata. In recognition of the re- 

 semblance of the larvae to holotrichs the group is sometimes placed as a 

 suborder in the order Holotricha. Another way to group the suctorians 

 and ciliates is to place them in a subphylum, the Ciliophora, separate 

 from the other protozoan classes. 



52. Class Sporozoa 



The Sporozoans are a large group of parasitic protozoa, some of 

 which cause such serious diseases as coccidiosis in poultry and malaria 

 (Fig. 6.1) in man. Neither locomotor organelles nor contractile vacuoles 

 are present. Nutrition is saprozoic, nutrients from the host being ab- 

 sorbed directly through the cell wall. Most sporozoans live as intracellu- 

 lar parasites within the host cells during the growth phase of their life 

 cycle. 



The cycle of cell division indicated in Chapter 6 for Plasmodium 

 is common in the class. The infecti\e spore matures as a feeding animal 

 or trophozoite. It then divides by multiple fission into a number of 

 young that infect new cells of the same host and mature as more tro- 

 phozoites. Eventually, however, some trophozoites fail to divide and 

 instead undergo metamorphosis to sexual forms. The females become 

 eggs, while the males divide by multiple fission into many sperm. In 

 some sporozoans the females also divide to form a number of eggs. After 

 fertilization the new individuals grow and divide by multiple fission 

 into a number of spores— individuals able to infect new hosts. The 

 spores of most sporozoans are encapsulated to withstand the dryness of 

 the external world. In blood parasites, however, such as plasmodia, the 



