Parasitic animal organisms are distributed through various 

 natural phyla, the chief of which are: 



(1) Protozoa, or Unicellular animals; 



(2) Platyhelminthes, or Flatworms; 



(3) Nemathelminthes, or Roundworms; 



(4) Annulata, or Segmented worms; 



(5) Arthropoda, or Joint-footed animals. 



PROTOZOA. 



The Protozoa are the lowest of animal organisms in the sense 

 that the functions and specializations of the individual are not 

 expressed by aggregation of mutually dependent cells. Four 

 natural classes are distinguished, viz., 



(a) Rhizopoda. Organisms with protoplasmic extensions of 

 the cell, pseudopodia. 



(b) Mastigophora. Organisms with one or two large flagella 

 for locomotion. 



(c) Infusoria. Organisms with the cell surface occupied 

 uniformly or in part by cilia, the latter sometimes (Tentaculifera) 

 present only in the embryo. 



(d) Sporozoa. Organisms specialized as parasites and having 

 partition cells or spores, produced in large numbers. 



Among the Rhizopoda are forms resembling ordinary amoebae, 

 but living in the human alimentary canal, especially the large 

 intestine. One of these, Endamoeba coli, lives as a commensal 

 in the first portion of the large intestine while E. histolytica {Amoeba 

 dysenteriae) is the causative agent of amoebic dysentery, character- 

 istic of the warmer zones, associated with inflammatory conditions, 

 haemorrhage and ulceration of the intestinal wall from penetration 

 and destruction of the mucosa. Both organisms have sporulation 

 stages suggesting those of Sporozoa. Other species of amoebae are 

 sometimes found on the oral, pulmonary and urino-genital surfaces. 



The parasitic Mastigophora are commonly differentiated as 

 Trypanosomes and Herpetomonads, and the Spirochaetes 

 and Treponemata are cometimes included. 



Trypanosomes are elongated spindle-shaped organisms, with a 

 longitudinal undulating membrane and an accessory parabasal 

 body giving origin to a flagellum which is closely associated with 

 the undulating membrane. They are blood-parasites occurring in 

 a great variety of vertebrates. Many appear to be harmless to the 

 host, the latter in all probability having become immune to their 

 presence. There is usually a secondary invertebrate host. 



Trypanosoma lewisi from the blood of the rat has been thoroughly 



