MONOCELLULAR ANIMALS-THE PROTOZOA 



125 



cells, although some live in colonies, such 

 as Symira (Fig. 7-17) and Codosiga (Fig. 

 7-16). The latter form possesses a collar at 

 the anterior end of each cell from which the 

 flagellum emerges. With this single excep- 

 tion, collared cells such as these are found 

 only among the sponges. Food is brought to 

 the walls of the cell by the flagellum, then 

 taken into the cell body to form a food vac- 

 uole. 



Parasitic flagellates. There is a wide va- 

 riety of flagellates that have made their way 

 into the body cavities and the blood streams 

 of almost every group of animals. They are 

 particularly common in the blood of verte- 

 brates, including man. The most common 

 offenders in this respect are the trypano- 

 somes (Fig. 7-18), tiny leaf -like, elongated 

 cells. A single flagellum, which lies at the 

 outer edge of the membrane, undulates as 

 the organism is propelled through the body 

 fluids of the host. Frequently the parasite is 

 transmitted from one vertebrate to another 

 by means of an intermediate host, either by 

 a blood-sucking insect or some other arthro- 

 pod. Some of the diseases caused by this 

 group of parasites are African sleeping sick- 

 ness and kala azar, an oriental disease 

 caused by Leishmania donovani. 



Kala azar has had devastating effects on 

 the populations of North China, various 

 parts of India, the Sudan, and South Amer- 

 ica. It has occurred primarily in the past, al- 

 though even today it is rampant in many 

 sections of the world. With the advent of 

 knowledge concerning epidemiology, diag- 

 nosis, and treatment, many of the evil ef- 

 fects of this disease have been greatly 

 lessened. 



Leishmania is a tiny ( 2-4 microns ) ovoid 

 parasite which attacks the cells of almost 

 all of the tissues of the body, particularly 

 the large cells of the tissues lining the circu- 

 latory system, both blood and lymphatic. 

 Upon entering the cells it multiplies (Fig. 

 7-19) until eventually the host cell bursts 

 and the released parasites attack other cells. 

 Some enter the blood stream where thev are 



Fig. 7-18. These tiny leaf-like trypanosomes live in 

 the blood of vertebrates and other animals. This is a 

 blood smear showing the parasites among the red 

 blood cells. 



picked up by the intermediate host, the 

 sand fly (Phlebotomiis) . In the gut of this 

 insect they become flagellated and change 

 considerably in shape. When the insect 

 bites another person, some time after it has 

 received the parasite, the flagellated fonns 

 are injected directly into the blood where 

 they attack the lining cells of the blood ves- 

 sels and the cycle is complete. Like most 

 blood-sucking insects, the sand fly intro- 

 duces a small amount of saliva, which has 

 an anticoagulating effect on the blood. If 

 this were not the case, a blood clot would 

 shortly interrupt the anticipated meal. 

 Therefore, the parasite is inadvertently in- 

 troduced along with the saliva, through no 

 "intent" on the part of the sand fly. 



The disease runs its course in a matter of 

 months or several years, frequently ending 

 in death. It has been shown that the flies 

 also bite dogs, which in turn act as reser- 

 voirs for the disease. So the problem of 

 eradication consists not only of preventa- 

 tive measures and treatment of infected 

 persons, but also control of the dog popu- 

 lation of any affected community. The most 



