REPRODUCTION AND THE LIFE CYCLE 



193 



tion considered in the preceding chapter. The spores are usually pro- 

 tected by thick and tough membranes, and are distinguished from all 

 other sporozoan spores by the presence of spirally wound threads con- 

 tained in two to four polar capsules. They are often ornamented in 

 some way and are always in the form of two valves, which meet in a 

 suture representing the line of splitting when the spores germinate 

 (Fig. 20, G, K, p. 64). The polar capsules are variously arranged in 

 the spore, and the usual interpretation of the thread is that originally 

 given by Thfelohan ('92), that they are for the purpose of anchoring 

 the spore in the lumen of the digestive tract. The most curiously 



Spores of actinomyxidse. (After Caullery and Mesnii.) A, Hexactinomyxon psam- 

 moryctis (after Stole), X 450; B, Spheractinomyxon stolci (after Caullery and Mesnii), X 900; 

 C, Triactinomyxon ignotum, Stol6, X 250; D, Triactinomyxon ignotum, spore-bearing part of 

 same enlarged (after Leger), X 900; E, Synactiuorayxon tubificis, Stole, X 900. In A,B,D, 

 and E, the evaginated spiral filaments are shown. 



ornamented of all spores are those of the actinomyxidse, where long 

 processes and curiously placed polar capsules and sporozoites are 

 characteristic (Fig. 82). 



3. Exogenous Life of Protozoan Parasites. — By exogenous life of 

 parasites is meant here the life outside of the usual host, whether this 

 is the primary or "intermediate" host. It is the most critical period in 

 the entire life history of a parasite, and a successful outcome is depen- 

 dent upon several factors, the most important being: (a) dissemination 

 of the spores, and (b) infection of new hosts, the latter factor in 

 particular having given rise to the most diverse adaptations. 

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