MORPHOLOGY AND CLASSIFICATION 155 



esses to serve for locomotion and also to surround and engulf 

 solid food particles. The two genera, Amoeba and Entamaeba, 

 are of chief est interest. The organisms are masses of protoplasm 

 containing a nucleus, food granules and sometimes vacuoles, 

 and surrounded by a slightly denser more hyaline layer of ecto- 

 plasm. The members of the genus Amoeba are free-living 

 saprophytic forms, while those of Eniamosba are parasitic. 

 Multiplication occurs by fission after a more or less complex di- 

 vision of the nucleus. Multiple division also occurs, more es- 

 pecially in an encysted condition, and subsequent to a possible 

 autogamic fertilization. 



Sporozoa. The third class, Sporozoa, is made up entirely 

 of parasitic forms, which at some stage in their life history multiply 

 by division into numerous daughter cells, which are enclosed in a 

 protective envelope to form a spore. The spores serve to dis- 

 tribute the species to other hosts. In cases where there are special 

 adaptations for distribution, as for example by means of inter- 

 mediate hosts, the protective envelope may be absent. An enor- 

 mous number of parasitic micro-organisms are included in this 

 group. The genera of greatest present interest from the patho- 

 logical point of view are Eimeria (Coccidium), Plasmodium 

 Babesia (Piroplasma) and Nosema. 



The Coccidia. Eimeria includes a number of intracellular 

 parasitic forms, perhaps better known as coccidia. The small 

 parasite resulting from asexual division is called a merozoit. It 

 is somewhat spindle-shaped and 5 to IOJJL long. This merozoit 

 penetrates an epithelial cell of the host, grows at the expense of 

 the cell to a spherical mass 20 to 50;* in diameter, and eventually 

 divides into numerous (sometimes as many as 200) merozoits, 

 which become free by rupture of the host cell. Besides this asexual 

 mode of multiplication, there is also a sexual cycle. Some of the 

 growing parasites do not divide into merozoits but become differ- 

 entiated into male and female cells (gametocy tes) . The male 

 gametocyte gives rise to a large number of elongated motile micro- 

 gametes, one of which approaches and penetrates the ripened 



