PROTOZOA l6l 



number of these daughter individuals corresponds to the number of 

 nuclei present. This process is known as schizogony. 



An alternation of hosts frequently occurs, and in such cases one 

 stage of the life-cycle may be passed in an invertebrate and another in 

 a vertebrate animal. 



Order Cocgidia 



In temperate climates Coccidia cause a greater loss to domestic 

 poultry, pigeons and g^me birds than any other group of Protozoa. 

 They are also common parasites of wild birds. Shipley pointed out 

 that the name is somewhat misleading since the public are apt to think 

 of a Coccidium as a bacterium or coccus^ whereas it no more resembles 

 this organism than a crocodile resembles a crocus. The best known 

 family is the Eimeridae (Fig. 2, a) which occurs in birds, mammals, 

 reptiles, amphibians, fish and arthropods. The whole of the growth 

 period of these parasites takes place within the cytoplasm of a host cell. 

 The oocysts are discharged in the droppings of infected birds, and may 

 contaminate food and water. If ingested by another bird while eating 

 or drinking, the oocysts pass into the duodenum where their thick 

 resistant wall is dissolved and the sporocysts are liberated. Each of 

 these sporocysts in turn sets free two active motile sporozoites which 

 bore into cells lining the intestine. Here they grow at the expense of 

 the host tissue. ^Vithin these epithelial cells, multiplication by schi- 

 zogony occurs repeatedly. The daughter individuals known as mero- 

 zoites eventually escape into the lumen of the intestine and from there 

 invade new host-cells. After several of these asexual cycles the resulting 

 merozoites become differentiated into ovoid macrogametes (female 

 cells) and flagellated microgametes (male cells). Each type develops in 

 a separate cell of the host. Copulation and fertilisation take place by a 

 liberated male cell penetrating a female cell; a resistant wall is formed 

 round the fertilised cell or zygote which now becomes an oocyst and 

 bursts out once again into the lumen of the intestine. It is, however, 

 incapable of further development until it is voided with the bird's 

 faeces. Conditions in the outside world are favourable and after some 

 time the single cell within the oocyst divides into two or four spores 

 (sporocysts). The oocysts have then reached the so-called infective 

 stage and if swallowed, are capable of infecting another host. 



