INTRODUCTION TO THE PROTOZOA 



21 



folds in the surface membrane and an 

 ultramicro^scopic network of fibrils about 

 50 to 200 A in diameter in the ectoplasm 

 of the trophozoites of Gregarina rigida 

 from the grasshopper. They believed that 

 gliding is probably accomplished by move- 

 ment of the body surface in contact with 

 the substrate and that the mucus which is 

 secreted may possibly provide a suitable 

 surface for locomotion. Ludvik (1958) 

 observed superficial, longitudinal fibrils 

 in electron micrographs of Sarcocystis 

 tenella. However, a definitive explana- 

 tion of the mechanism of gliding still 

 eludes us. 



EXCRETORY ORGANELLES 



Excretion in the Protozoa is either 

 thru the body wall or by means of a con- 

 tractile vacuole which may be simple or 

 may be associated with a system of feeder 

 canals or vacuoles. Contractile vacuoles 

 are probably more important as osmoreg- 

 ulatory organelles than for excretion. 

 They maintain water balance by removing 

 excess water from the cytoplasm and 

 passing it out of the body. They are found 

 in fresh-water protozoa but are absent in 

 most marine and parasitic protozoa. How- 

 ever, some of the latter, including Balan- 

 tidiuni and trypanosomes, contain them. 



ORGANELLES ASSOCIATED 

 WITH NUTRITION 



OTHER ORGANELLES 



Nutrition among the protozoa may be 

 of several types. Rather elaborate clas- 

 sifications have been proposed by some 

 authors, but three types are sufficient for 

 our purposes. In holophytic nutrition, 

 which is characteristic of the phytoflagel- 

 lates, carbohydrates are synthesized by 

 means of chlorophyll which is carried in 

 chroiuatophores , which vary consider- 

 ably in size, shape and number. 



In holozoic nutrition, particulate food 

 material is ingested thru a temporary or 

 permanent mouth. A temporary mouth is 

 formed by amoebae when they engulf their 

 food. A permanent mouth is a cytostome. 

 It may be simple or it may lead into a 

 cytopharynx. In many ciliates the area 

 around the cytostome forms a peristome, 

 and there may be a number of other spe- 

 cialized structures associated with it. 

 Particulate food passes into a. food vacuole 

 in the cytoplasm, where it is digested. 

 The indigestible material may be extruded 

 from the body either thru a temporary 

 opening or thru a permanent cytopyge. 



In saprozoic nutrition, no specialized 

 organelles are necessary, nutrients being 

 absorbed thru the body wall. This type is 

 found in many protozoa, and may be pres- 

 ent along with holophytic or holozoic nu- 

 trition. 



Protozoa have many other specialized 

 organelles which are found in different 

 groups. These will be described in the 

 appropriate places below. 



REPRODUCTION AND LIFE CYCLES 



Reproduction in the Protozoa may be 

 either asexual or sexual. The commonest 

 type of asexual reproduction is binary 

 fission, i.e., each individual divides into 

 two. The plane of fission is longitudinal 

 in the flagellates and transverse in the 

 ciliates. Cytoplasmic division follows 

 nuclear fission and separation of the 

 daughter nuclei. Vesicular nuclei and 

 micronuclei divide mitotically; macronu- 

 clei divide amitotically. 



Multiple fission or schizogony is 

 found mostly in the Telosporasida. In this 

 type of fission, the nucleus divides several 

 times before the cytoplasm divides. The 

 dividing cell is known as a schizont, 

 agamont or segmenter, and the daughter 

 cells are merozoites or schizozoites. 

 Nuclear division, again, is mitotic. 



A third type of asexual division is 

 budding. In this process, a small daugh- 

 ter individual is separated off from the 



