PROTOZOA 171 



Other animals. Reproduction is usually by binary fission, the animal 

 dividing into two by splitting along the longitudinal axis. 



In the case of parasitic forms the life-cycle may involve development 

 in an intermediate host. 



Order Protomonadida 



As far as birds are concerned the most important flagellate parasites 

 are concentrated in this order. Of these the best known belong to the 

 family Trypanosomidae (Plate XXV), which are parasitic in verte- 

 brates, invertebrates and certain plants with a milky "juice" in their 

 stem and leaves. The only genus recorded from British birds is 

 Trypanosoma (see tail-piece of Chapter 9). Under the microscope 

 these parasites superficially resemble little fish — with a long, un- 

 dulating crest in place of a dorsal fin. The fi.agellum runs along the 

 outer margin of this membrane and projects beyond it as a free lash. On 

 a slide some species can be observed wriggling sluggishly among the 

 blood corpuscles while others dart about like lively minnows. Try- 

 panosomes have no mouth (cytostome) and their food is absorbed in 

 liquid form through the cell membrane. The life history of the species 

 from birds has not been fully worked out. Like the majority of try- 

 panosomes, they occur chiefly in the blood, but in some cases they have 

 also been found in the bone marrow and other tissues of the vertebrate 

 host. Generally the birds appear to be unharmed by their presence but 

 in the laboratory when unusual hosts are used death may follow an 

 artificially produced infection 



The genus Trypanosoma is very common in birds and has been 

 recorded from over 200 species. It is customary to name each one of 

 these as if it represented a host-specific trypanosome. Thus, for example, 

 the one found in the chaffinch is named T.fringillinarum. In all proba- 

 bility the same species occurs in a number of diflferent hosts as the 

 organisms in question are very variable (polymorphic). In Britain 

 Trypanosoma has been found in the rook, jackdaw, yellow-hammer, 

 chaffinch, linnet, blackbird, jay, thrush, house-martin and swallow. 

 In Germany it has been recorded from many other birds on the British 

 list, and is often present in the blood of nestlings only a few days old. 



Most trypanosomes are transmitted by invertebrate hosts. For 

 instance, a trypanosome of sheep is carried by the sheep ked (a louse- 

 fly), one from the rat by fleas, one from the tortoise by leeches, another 

 from the camel by horse flies and the most famous of all trypanosomes. 



