150 



PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



grouse, pheasants, and other common birds, as well as a 

 large number of Invertebrates. They are intracellular 

 parasites. Gregarines are common gut parasites of most 

 animals, at first intracellular, later becoming free in the 

 gut cavity. The Spirochaetes form a group of often ex- 

 tremely minute spiral organisms which some regard as 

 Bacteria, others as Protozoa. Some of them are among the 

 most formidable parasites of man. Treponema {Spiro- 

 chceta) pallidum is the cause of syphilis, one of the 

 heaviest taxes on civilisation. Many Spirochastes are 



Fig. 70. — Colonial Infusorian — Ophrydium sessile. — After 

 vSaville Kent. (Enlarged 100 times.) 



The individuals are embedded in a jelly-like matrix (/.). M., Mouth ; B., 

 band round oral disc ; C, circumoral cilia. 



blood parasites, e.^. S. duttoni^ conveyed by a tick, is the 

 cause of African relapsing or tick fever. Another species 

 of Spirochaete, S. recurrenlis, is transmitted in more 

 northern countries in the haemocoele of the body-louse, 

 and causes what is termed " European " relapsing fever. 



General zoological interest. — The Protozoa illustrate, 

 in free and single life, forms and functions like those of the 

 cells which compose the many-celled animals. Typically, 

 they show great structural or morphological simplicity, but 

 great physiological complexity. Within its single cell the 

 Protozoon discharges all the usual functions, while in a 

 higher animal distinct sets of cells have been specialised for 



