2l8 THE PROTOZOA 



made to serve as a matrix for the chromatin. Chromatin may be concentrated 

 in a single mass so that the nuclear space looks like a vesicle with a central chroma- 

 tin mass (vesicular nucleus) or numerous chromatin grains may be scattered through 

 the nuclear space (granular nucleus). The centrosome, which presides over cell 

 division, is usually located just outside the nucleus. In some protozoa however 

 the centrosome is within the nucleus and is often seen inside of a karyosome and is 

 then called a centriole. The centrosome may also function over kinetic activities 

 (flagellar motion) and is then termed blepharoplast. 



Certain protozoa, as trypanosomes, show a differentiation of nuclei, the larger 

 trophonucleus governing the functions of general metabolism and the smaller kine- 

 tonucleus directing the motor activities. Infusoria have a larger macronucleus 

 which contains vegetative chromatin and a smaller micronucleus which contains 

 reserve reproductive chromatin. 



Reproduction of protozoa may be by fission, when the nucleus and cytoplasm 

 divide into two by simple division. 



When the nuclei divide into a number of daughter nuclei, which is followed by 

 multiple division of the cytoplasm, we have sporulation. 



Instead of fission we may have sexual reproduction or conjugation (zygosis). 

 Here the nuclei of the separate sexual individuals (gametes) are termed pronuclei 

 and the product of their fusion a synkaryon. 



Where a single cell has division of its nucleus with subsequent fusion of these 

 daughter nuclei to form a synkaryon the process is termed autogamy. 



If two similar cells conjugate the term is isogamy; if dissimilar as the macro- 

 gametes and microgametes of malaria, anisogamy. 



The process of sexual union is termed syngamy and is of two kinds (i) when 

 the two gametes fuse completely or copulation and (2) when they remain separate 

 and only exchange nuclear material or conjugation. 



The structures of protozoa concerned in movement, metabolism, etc., are termed 

 organelles. Of the former, pseudopodia, flagella, cilia and myonemes (contractile 

 fibrils which give support to the body cell of certain protozoa) may be given and 

 food vacuoles and contractile vacuoles of the latter. The contractile vacuole 

 which is probably an excretory organelle is absent in almost all parasitic protozoa. 

 It is however present in ciliates. 



RHIZOPODA (SARCODINA). 



In this class of protozoa the pseudopodia serve the double purpose 

 of nutrition and locomotion. These protoplasmic extensions may be 

 quite broad or very narrow the lobose and the reticulose. 



As a rule, the thicker the pseudopod the more rapid the movement. 

 Some rhizopods have hard shell-like coverings which are secreted in or 

 on the ectosarc. These skeletons have openings through which the 

 pseudopods project. The pseudopodia may be made up only of ecto- 

 plasm or both ectoplasm and endoplasm may take part. Amoeboid 

 movement always starts in the ectoplasm. In addition to the nucleus, 



