PROTOZOAN PARASITES 



291 



minute grain or pair of granules (diplosome) lying typically outside 

 the nucleus close to the membrane. (7) The archoplasm is the clear 

 protoplasm which sometimes surrounds the centrosome. (8) The 

 rhizoplast is the portion of the fLagellum (when present) which 

 penetrates into the cytoplasm. The flagellar apparatus is described 

 under the heading ' Mastigophora ' in the next chapter. (9) The 

 vacuoli may be either contractile vacuoles which are considered 

 to be respiratory and excretory in function, or the food vacuoles, 

 which begin with a globule of water taken in with the food. Into 

 this vacuole an acid is secreted from the cytoplasm and digests the 

 food, which is then absorbed. Then the vacuole with the undigested 

 food travels to the periphery, and the waste product is extruded. 



Protozoa generally have some power of movement by pseudo- 

 podia, cilia, or flagella, but under unfavourable circumstances they 

 may lose this power, and, becoming quiescent, surround them- 

 selves with an envelope and become encysted. Unfavourable 

 circumstances are lack of food, desiccation, irritating chemical 

 substances, and unusual surroundings. 



Life-History. — ^Reproduction in the protozoa takes place asexually 

 or sexually. As long as conditions are favourable — i.e., there is 

 abundance of food— protozoa reproduce asexually by one of the 

 following methods: (i) binary fission; (2) gemmation; and (3) spore- 

 formation. 



I. Binary Fission. — In binary fission there is first division of the 

 nucleus, which sometimes takes place by amitosis, followed by that 

 of the cytoplasm. Then the parasite divides into two more or less 

 equal halves (Fig. 42). The other methods of nuclear division are 

 by chromidial fragmentation or by mitosis, of which there are three 

 types — {a) Promitosis, (b) Mesomitosis, (c) Metamitosis. In chro- 

 midial fragmentation the nucleus breaks up into minute chromidia, 

 which eventually collect into two new nuclei. In Promitosis a pro- 

 karyon type of nucleus divides by the centrosome splitting into two, 

 and giving rise to the central thread (centrodesmose) of the spindle. 

 Then the karyosome divides by constriction, and the achromatic 

 spindle is formed from the framework of the nucleus, and lies 

 between the two separating karyosomes, with the centrodesmose in 

 the middle. Then the chromosomes appear formed from the peri- 

 pheral chromatin as well as from that of the karyosome. 



There are two types of Promitosis — the simpler, in which no 

 equatorial plate is formed, and the chromosomes are merely scat- 

 tered along the spindle, and finally gather at opposite poles to form 

 the daughter nuclei; and the more advanced, with an equatorial 

 plate of chromosomes which may divide by either an equating or 

 a reducing division. In Promitosis the nuclear membrane is neg- 

 ligible, while the whole process is confined to the nuclear area. In 

 mesomitosis, which takes place in a nucleus in which the karyosome 

 is reduced and in which there are more chromatic particles in the 

 body of the nucleus, perfect karyomitotic figures are formed, but 

 the whole process takes place inside the nuclear membrane. In 



