1 64 THE SPOROZOA 



L. Pfeiffer in. the intestines and faeces of various birds. 1 A parallel case 

 is that of the Coccidium infecting the centipede Lithobius, the spores of 

 which, if swallowed by another animal, such as a wood-louse, pass unaltered 

 through it (see p. 221). If this suggestion be correct, it is easy to under- 

 stand that any worm-eating bird would be continually scattering spores 

 of Monocystis on the ground, where they would wash down into the soil 

 and be swallowed very easily by worms again. There is, however, no 

 direct evidence bearing upon the mode of dissemination of the spores, 

 and the above suggestion must be regarded merely as a more or less 

 probable surmise. 



When the spores have reached the digestive tract of their new host, 

 and the sporozoites have been liberated there, the question arises how 

 they reach the sperm-sacs. This problem, however difficult to solve, is 

 by no means one peculiar to the Monocystis of the earthworm. In many 

 other Sporozoa we have instances of parasites affecting some particular 

 organ, which invade the body in the first place from the digestive tract. 

 It must be assumed that the sporozoites have in some way the power of 

 selecting the particular organ they affect, and of migrating through the 

 body of the host in order to reach their specific habitat. Probably they 

 make use of vascular or lymphatic channels in order to arrive at their 

 destination. 



General Characters of the Sporozoa. From the above account 

 of Monocystis it is seen that the life-history of a typical Sporozoon 

 is a single cycle, which may be summed up in the following 

 way : 2 



Sporozoite->-Trophozoite->-Gametocyte (Sporont) x n Gametes \ , 

 Sporozoite->-Trophozoite->-Gametocy te (Sporont) x n Gametes / "" 



= n Zygotes (Sporoblasts)->-n Spores x 8)i Sporozoites. 



The life-cycle may further be divided into three main periods. 



First, the period of growth, during which the minute sporo- 

 zoite grows by absorption of nutriment from the host into the 

 sporont. 



Secondly, the period of proliferation, accompanied by conjuga- 

 tion, and resulting in the formation of a large number of germs, 

 destined to spread the species. 



Thirdly, the period of rest, during which the parasitic germs 

 pass out from the host into the outer world, to effect, if fortune 

 favour them, the passive infection of a new host. 



In Sporozoa, considered generally, the life-history is similar in 

 the main to that described above, but exhibits, in different forms, 

 variations of every kind, in the direction either of greater or 

 of less complexity. The deviations from the selected type may 



1 Fide Wasielewski [7], p. 26. 



2 In this and in all subsequent formulae of sporozoan life-histories an arrow is 

 used to mean "becomes" or "grows into" ; the sign x to indicate a distinct cell- 

 generation, a multiplication of individuals of any kind ; and a bracket with the 

 sign + to denote the occurrence of zygosis or true conjugation and fusion of gametes. 



