86 THE INVERTEBRATA 



(i) Schellackia (suborder Coccidia), parasitic in the gut of a 

 tortoise, leaves the gut epithelium after schizogony and completes its 

 cycle in the subepithelial tissues. In order to reach a new host it has 

 therefore to rely on transference by a carrier instead of passing out with 

 the faeces. To accomplish this, the sporozoites enter blood vessels, 

 get into red corpuscles, and are sucked up by a mite. The blood- 

 sucker, however, does not inject the parasite into the new vertebrate 

 host, but is swallowed, so that the parasite infects the host through 

 the gut epithelium, in which its schizogony is still performed. 



(2) Lankesterella (suborder Coccidia), parasitic in frogs, passes its 

 whole cycle in the epithelioid lining of blood vessels, the sporozoites 

 being transferred, as in Schellackia, in red corpuscles, which are 

 sucked up by a leech. Infection is still through the gut of the verte- 

 brate, whose wall the sporozoites pierce on their way to the blood 

 vessels. 



(3) Haemoproteus (Haemosporidia), parasitic in birds, has its 

 schizogony alone in the blood vessel walls, the sexual part of the cycle 

 being remitted to the invertebrate host. The parasite enters the red 

 corpuscles not as a sporozoite but earlier, as the young stage of the 

 gamont, which grows up in the corpuscle. At the same time a change 

 in the mode of infection has taken place, the blood-sucker injecting 

 the sporozoites into the blood vessels of the vertebrate host. Thus the 

 parasite has completely abandoned the gut wall and become a true 

 blood parasite. 



(4) Plasmodium (Haemosporidia), the cause of malaria and ague in 

 man, is parasitic in the red blood-corpuscles of mammals and trans- 

 mitted by the mosquito Anopheles. Its schizonts (trophozoites), as 

 well as its gamonts, inhabit red corpuscles. 



The trophozoites of Plasmodium (Fig. 75) are amoeboid. In the 

 young stage they are rounded and each contains a large vacuole which 

 gives it the appearance of a ring. They undergo schizogony in the 

 red corpuscles, which then break up, setting free the schizozoites 

 (merozoites) and also products of the metabolism of the parasite which 

 cause fever. After some generations, gamonts similar to those of 

 Eimeria appear, but remain quiescent unless sucked up by a mosquito, 

 in whose gut the female gamont becomes a spherical macrogamete, the 

 male gamont throws off whip-like microgametes, and syngamy takes 

 place. The zygote becomes elongate and active (an ookinete), and 

 bores its way through the wall of the mosquito's stomach, on the 

 outside of which it becomes encysted (oocyst). Here its nucleus 

 divides and it breaks up into sporoblasts which in turn produce 

 spindle-shaped sporozoites. The oocyst now bursts, setting the 

 sporozoites free in the blood of the insect. They make their way 

 into the salivary glands and are injected with the saliva into a 



