RHIZOPODS 251 



secretion or modification of chro matin 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 chromatin mass (vesicular nucleus) or numerous chro- 

 matin 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. 



When appearing as a small granule at the base of the flagellar apparatus it is called 

 the basal granule. When there are extensions from it to the nucleus we have 

 rhizoplasts. 



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 macroga- 

 metes 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 rhizo- 

 pods 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 ectoplasm or both ectoplasm and endoplasm may take part. 



