308 BIOLOGY: GENERAL AND MEDICAL 



ishing frequency as the scale of life is ascended until, 

 when the vertebrates are reached, there is but a single 

 representative. 



The following systematic arrangement of the parasitic 

 forms is founded upon the excellent paper upon the sub- 

 ject in the New International Encyclopedia. 

 Phylum Protozoa. 



Class Rhizopoda. These include the amoeba, 

 of which many species are parasitic in the 

 alimentary tracts of many other animals. The 

 most important is the Entamoeba histolytica 

 Schaudinn, that causes tropical dysentery 

 in man. 



Class Infusoria. These are ciliated organisms 

 in some cases, like Balantidium coli of man, not 

 certainly parasitic, but perhaps merely com- 

 mensals. TrichodinaB is a parasite of the gills 

 and gill cavity of the frog; Opalina, of the 

 bladder and gut of the frog; Holophyra, an 

 epiparasite of certain fish; Ancistrum infests 

 the mantle cavity of certain mollusks; Ano- 

 phlophyra occurs in the intestines of certain 

 marine invertebrates. 



Class Mastigophora. These are flagellate or- 

 ganisms of which some are commensals, like 

 Trichomonas intestinalis, Trichomonas vagi- 

 nalis, Cercomonas intestinalis, and Megastoma 

 entericum, though it is not certain that the 

 latter is not a true parasite. They also in- 

 clude a number of alimentary and blood 

 parasites, such as Herpetomonas, and true blood 

 parasites, such as the Trypanosom.es, many 

 of which are dangerous or fatal parasites of 

 man and the lower animals. 

 Class Sporozoa. These form a large group of 

 intracellular parasites, some, as Coccidia, being 

 parasites of epithelial cells, others like, Plasmo- 

 dium malariae, blood parasites, and still others, 



