REPRODUCTION AND THE LIFE CYCLE 



193 



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

 tected bv thick and tough mcnihranes, 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 Thelohan ('92), that they are for the purpose of anchoring 

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



Fig. 82 



Spores of actinomyxidae. (After C.aullery and Mesnil.) ,1, Hexactinoinyxon psam- 

 moryctis (after Stolfi), X 450; B, Spheraotinomyxon stoici (after Caullery and Mesnil), X 900; 

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

 same enlarged (after Loger), X 900; E, Synactinomyxon tubificis, Stol6, X 900. In .-1, B, D, 

 and E, the evaginated spiral filaments are shown. 



ornamented of all spores arc tiiose of the actinomyxidic, where long 

 proce.s.ses and curiously placed polar capsules and sporozoites are 

 characteristic (Fig. <S2). 

 :i Exogenous Life of Protozoan Parasites. By cxogtMious life of 



parasites is incaiit licrc the ht'e outside of th(> usual host, whether this 

 is the j)riinary 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 imj)ortant being: (a) (Iis.seminati()n 

 of the .spores, and (}>) infection of new hosts, the latter factor in 

 p.'irtieular having given ri.se to the most diverse adapditions. 



