GENERAL ZOOLOGY 



Fig. 8.13. Trypanosoma gamlneme, the causative 

 organism of one form of African sleeping sicli- 

 ness. The trypanosomes are shown as they 

 appear in a smear of blood from an infected 

 individual; the circles represent erythrocytes 

 and indicate the relative size of the parasites. 



is an elongated cell with an undulating membrane along one side, bounded 

 laterally by an attached flagellum. The flagellum springs from a granule in 

 the posterior end of the cell and becomes free anteriorly. A representative 

 example is Trypanosoma oatnhiense, which causes one type of sleeping sickness 

 so dangerous to man in equatorial Africa. By the bite of the bloodsucking 

 tsetse fly, the stages of the parasite found in human blood are transferred to 

 the intestine of this insect, which serves as the intermediate host. Here a 

 series of reproductive stages occur; some 3 to 4 weeks later the parasites 

 appear in the salivary glands of the fly, from which they may again be trans- 

 ferred to the blood of man or other mammals. In the final stages of 

 parasitization, the trypanosomes invade the cerebrospinal fluid of the host, 

 inducing the stupor that characterizes the disease and finally ends in death. 

 Although these parasites produce a fatal disease in man, thev cause no 

 obvious ill eflects in other hosts among the larger African mammals. Such 

 a contrast in susceptibility may perhaps be explained on the theory that the 

 wild mammals in question have become tolerant of the infection or have 

 developed an immunity to its efllects, as a result of natural selection acting 

 upon very many generations. Man, on the other hand, has only compara- 

 tively recently come into contact with these parasites and is very susceptible. 



The Sporozoa 



The class Sporozoa contains only parasitic species. In correlation with this 

 mode of life, the locomotor and other structures necessarv in free-living ani- 

 mals are much reduced. The name Sporozoa ("seed animals") was given 

 because "seed-like" stages, or spores, are conspicuous in the life cycles of 

 these protozoans. Representative examples are species of the genus Mono- 

 cystis, which inhabit the seminal vesicles of earthworms. The full-grown 

 individual is an elongated cell with a single nucleus. This organism is capable 

 of a slow, gliding locomotion by local contractions and extensions of the cell, 

 but there are no complex locomotor structures or behavior. Monocystis is first 



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