178 PARASITISM 



organ is produced into hooks, etc., the attaching portion being known 

 as the epimeriic. The portion suspended from the cell in the lumen of 

 the organ may be further differentiated by septa of ectoplasmic origin 

 into an anterior and a posterior part, the former called the 'primite, 

 the latter, usually containing the nucleus, the deutomerite (Fig. 1, D, 

 p. 17). 



Other special adaptive structures brought about in the protozoan 

 cell, as a result of parasitism, are undoubtedly the protective capsules 

 which envelop the spores. When the parasite becomes sexually 

 mature it fuses with another cell in conjugation, and fertilization is 

 followed by spore formation. The spores thus formed do not reinfect 

 the same host, but, contained usually in the lumen of the digestive 

 tract or similar cavity of the body, they are finally carried to the outside 

 in one way or another with the waste matters. Here, were it not for 

 the protective coverings which they possess, they would soon be killed 

 by exposure, but, protected by resistant chitinous membranes, such 

 spores resist drying and retain their vitality until again taken into a 

 new host, usually by way of the digestive tract. Animals of gregarious 

 habits are particularly subject to protozoan infection, the spores 

 usually contaminating the food. In the intestine the germs of the 

 organisms are liberated from their coverings and make their way by 

 one means or another to the definitive locality where growth is possible. 

 The so-called "selection" of locality is a matter of mere passive 

 resistance on the part of the parasite, that part being "selected" 

 where they are not destroyed by the reactions of the host, and where 

 conditions of life are most satisfactory for novirishment and security. 



If the yoimg organism is a gregarine or coccidian, it makes its way 

 to the epithelial cells lining the digestive tract and grows to adult size. 

 Some forms penetrate the walls of the gut and get into the celom where, 

 as celozoic parasites, they grow to maturity. Coccidia remain in the 

 first cell-host until it is destroyed, such destruction allowing the para- 

 site to fall into the lumen of the organ, where fertilization occurs. 

 Coccidian infection, for this reason, is much more severe than gre- 

 garine infection, and may give rise to acute enteritis (e. g., Cyclospora 

 caryohjtica in moles). 



II. REPRODUCTION AND THE LIFE CYCLE. 



In common with the many-celled parasites, the protozoan forms 

 have acf juired varied and prolific means of multiplication, which may 

 differ in type at different periods of the life cycle. In the majority of 

 cases such multiplication may involve sexual processes, or it may 

 be entirely asexual, the former occurring at the end of the vegetative 

 life of the parasite, the latter, during the vegetative life, in the host. 



