II 



PHYLUM PROTOZOA 



87 



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 microgamates, the result being the 

 formation of an active spindle-shaped ookinete. This perforates the stomach 

 wall and comes to rest in the subjacent tissues. It then becomese encysted 

 and increases greatly in size, bulging out into the body-cavity (b e). The 

 contents of the cyst eventually become divided up (f, 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. Penetrating into the interior of coloured corpuscles they reach the 

 trophozoite condition. 



The Hasmogregarines, which may most conveniently be referred to here, are 

 Sporozoa which live, like the malaria parasites, in the coloured blood-corpuscles 

 of all classes of Vertebrates ; but which in the mature or trophozoite condition 

 are not amoeboid, retaining the Gregarina-like form, and are therefore to be 

 regarded as belonging to the Gregarinida. 



ORDER 4. MYXOSPORIDEA. 



This group includes a small number of genera which are ainceboid 

 in the trophozoite phase, and which reproduce continuously by 

 spore-formation during that phase (Fig. 67, A). Many nuclei are present 



FIG. 67. A, Myxidium lieberkiihnii, amoeboid phase; B, IWtyxobolus miilleri, 



spore with discharged nernatocysts (ntc.); C, spores (psorosperms) of a Myxosporidian ; 

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



in the amoeboid body, which may be of comparatively large size. The 

 spores (B) produced within the protoplasm of the trophozoite are provided 

 each with one or more bodies like the nematocysts of zoophytes and jelly-fish 

 [See Section IV]. Myxosporidea occur as parasites mainly of fishes 

 and amphibians, but very many occur in various groups of Invertebrates. 

 "Pebrine," the destructive silk- worm disease, is due to the presence of a 

 Sporozoan belonging to this order. A good example of the order is Myxidium, 

 found in the urinary bladder of the pike. 



