380 MICROBIOLOGY 



Class II. Neosporidia. Sporozoa in which the entire cell is not 



used at one time in forming spores, the latter developing 



while ordinary processes are carried on. 



Order 4. Myxosporidia. Neosporidia with spores containing 

 polar capsules and anchoring threads. Here belong 

 several genera of note, in that serious epidemics of 

 lower animals are caused by them, e. g., Nosema caus- 

 ing pebrine diseases in silkworm, Myxobolus, Myxidium, 

 etc. 



Order 5. Sarcosporidia. Neosporidia in which the initial 

 stages are passed in muscle cells of vertebrates. Cysts 

 are formed with double membranes in which kidney- 

 shaped reproductive elements are produced. The one 

 genus occasionally parasitic in man is Sarcocystis. 



Protozoan infection. The exogenous life of protozoan 

 parasites is the life of the parasite outside of the usual host 

 whether it is the primary or "intermediate" host. This is the 

 most critical period in the entire life cycle of the parasite and, 

 the success of infection depends largely upon two of several 

 factors, (1) dissemination of the spores and (2) infection of 

 new hosts. In regard to the latter, many and diverse adapta- 

 tions have arisen. The same general conditions hold with 

 pathogenic protozoa affecting animals as with those attacking 

 man. The environmental conditions which protozoan parasites 

 have to meet and overcome are well stated in a principle by 

 Manson 5 and are quite as applicable for protozoa pathogenic 

 for animals as they are for those affecting man. "The patho- 

 genic protozoa are responsible probably for a very large num- 

 ber of diseases. Many appear to be able to pass directly from 

 host to host, unaffected apparently by the atmospheric condi- 

 tions they encounter on the passage; that of smallpox and of 

 most of the exanthematous fevers probably belong to this 

 category. Others, on the contrary, demand special climatic 

 conditions. Such are the germ of scarlet fever, which does not 

 spread in the tropics, and the germ of dengue which, con- 

 versely, does not spread in cold climates. That of the first is 



5 Albutt and Rolleston's System of Medicine, Vol. II, Part II, 

 1907. 



