II 



PHYLUM PROTOZOA 



89 



In some of the Coccidiidea this life-cycle is modified in various ways, as, 

 for example, by the omission of schizogony the trophozoites in such a case 

 developing directly into gametocytes. 



ORDER 3. HJEMOSPORIDEA. 



These are Sporozoa which in the trophozoite condition live as parasites in 

 the interior of the coloured blood-corpuscles of all classes of Vertebrates, but 

 are occasionally found in other cells. In man and in some other mammals and 

 in certain birds it has been found that then: presence is the cause of various 

 feverish affections. The various forms of malaria in man have been proved to 

 be due to the presence in the blood-corpuscles of the patient of parasites 

 belonging to this order. The malaria parasites, the history of which has been 

 carefully worked out, pass through a life-cycle comparable to that of Coccidium 

 described above. In the trophozoite stage (Fig. 67, A-G) they live as 

 amoaboid intracellular parasites in the interior of the coloured corpuscles of 

 their host. Here they multiply by schizogony the products (nierozoites) 

 entering other corpuscles. Some of the merozoites when they become estab- 

 lished in the interior of the corpuscles develop into rounded or crescentic 

 bodies which become the gametocytes (H, N, 0, T). In order that the life- 

 cycle may be completed, it is necessary that the parasite at this stage should 

 be taken into the interior of a second or intermediate host. In the case of the 

 parasite of human malaria the intermediate host is a mosquito of the genus 

 Anopheles. On the mosquito drawing up a drop of the blood of a malaria 

 patient, all stages of the parasite that occur in it are destroyed by the digestive 

 juices of the insect with the exception of the gametocytes ; these survive and 

 form gametes in the stomach of the mosquito. Each male gametocyte gives 

 rise to a number of slender filamentous microgametes (sperms, P, S) and each 

 female gametocyte forms a single megagamete (ovum). After maturation 

 (U-W) the megagamete is fertilised (x) by one of the actively-moving micro- 

 gametes, the result being the formation of an active spindle-shaped ookinete 

 or vermicule. This perforates the stomach-wall and comes to rest in the 

 subjacent tissues. It then becomes encysted and increases greatly in size, 

 bulging out into the body-cavity (b-e). The contents of the cyst eventually 

 divide up (/, g) into a large number of long, narrow sporozoites. When 

 the cyst becomes ruptured into the body-cavity, these find their way 

 to the salivary glands 

 (h), and thence they may 

 readily be transferred to 

 the blood -system of a 

 human being when the 

 mosquito bites. Pene- 

 trating into the interior 

 of coloured corpuscles 

 they reach the tropho- 

 zoite condition. 



The Hsemogregarines, 

 which may most con- 

 veniently be referred to 

 here, are Sporozoa which 

 live in the coloured 

 blood-corpuscles of all 

 classes of Vertebrates, 

 but chiefly in Fishes, Am- 

 phibians, and Reptiles ; 

 but which, unlike the malaria parasites, in the mature or trophozoite con- 

 dition are not amoeboid, retaining the Gregarina-like form. Transference from 

 one host to another is effected by means of various blood-sucking Invertebrates. 



FIG. 68. A, Myxidium lieberkuhnii, amoeboid phase ; 

 B, Myxobolus miiUeri, spore with discharged nemato- 

 cysta (ntc.) ; C, spores (psorqsperms) of a Myxosporidian ; 

 ntc. nematocysts. (From Biitschli's Protozoa.) 



