l60 FLEAS, FLUKES AND CUCKOOS 



attaches itself firmly to the surface of the intestinal cells of vertebrates, 

 including birds such as herons, shrikes and avocets. In certain groups 

 the mouth (cytostome) is frequently missing although this organ is present 

 in related free-living forms. Cyst formation is also characteristic of 

 parasitic Protozoa, such as Eimeria from the grouse. Cysts provide 

 the chief means of transference from host to host, since they protect the 

 enclosed parasite against the influence of the external environment and 

 resist the action of the digestive juices of the stomach. These properties 

 enable the protozoon to gain access to the internal organs of the bird 

 when swallowed with food and water. An enhanced power of reproduc- 

 tion involving multiple fission instead of the more usual binary fission 

 is also a typical feature of the parasitic forms. Complicated life-cycles, 

 with alternating vertebrate and invertebrate hosts, are found in many 

 Sporozoa and Flagellata from birds. The development of host specifi- 

 city and increased virulence are also characteristic of numbers of these 

 organisms — two phenomena which have been considered in previous 

 chapters. The parasitic Protozoa of birds (Fig. 2) belong to the three 

 classes, Sporozoa, Mastigophora and Rhizopoda, of which by far 

 the most important types are those grouped in the Class Sporozoa. 



Class Sporozoa 



The Sporozoa are exclusively parasite and live and feed in the cells 

 and body fluids of other animals. In the absence of a mouth the food 

 — which is in solution — passes into the body in liquid form and is 

 absorbed by osmosis. The proteid which is in solution is absorbed in 

 liquid form. During much of their life-cycle Sporozoa lack organs of 

 locomotion. They are also characterised by a highly specialised type 

 of reproduction. At some stage of their development they produce 

 cysts (oocysts) within which the infective forms called sporozoites are 

 found. In the Coccidia these are carried to new hosts within this 

 protective envelope. 



The life-cyle is complicated, with alternating sexual and asexual 

 phases. In the asexual phase, instead of simple division into two 

 separate individuals the nucleus of the growing parasite, known as the 

 trophozoite, divides repeatedly. Each resulting nucleus becomes 

 surrounded by a portion of the cytoplasm, and the body of the parasite, 

 now known as a schizont, breaks up into daughter individuals. The 



