loo PROTOZOA. 



parts of the cell-substance, {b) The production of a small 

 bud from a parent cell is not uncommon, and some Rhizo- 

 pods {e.g., Arcella, Pelomyxa) give off many buds at once. 

 \c) Commoner, however, is the definite and orderly process 

 by which a unit divides into two — ordinary cell-division. 

 {d) Finally, if many divisions occur in rapid succession or 

 contemporaneously, and usually within a cyst enclosing the 

 parent cell, i.e., in narrowly limited time and space, the 

 result is the formation of a considerable number of small 

 units or spores. Rupture, budding, division, and rriultiple 

 division or spore-formation are the methods by which the 

 Protozoa multiply. In the great majority of cases each 

 result of division is seen to include part of the parent 

 nucleus. 



A many-celled animal multiplies in most cases by liberat- 

 ing reproductive cells — ova and spermatozoa — different 

 from those which make up the "body." A Protozoon multi- 

 plies by dividing wholly into daughter cells. This difference 

 between Metazoa and Protozoa in their modes of multiplica- 

 tion is a consequence of the difference between multi- 

 cellular and unicellular life. Each part of a divided Pro- 

 tozoon is able to live on, and will itself divide after a time, 

 whereas the liberated spermatozoa and ova of a higher 

 animal die unless they unite. 



By sexual reproduction, we usually mean (a) the liberation 

 of special reproductive cells from a "body," and (/') the 

 fertilisation of ova by spermatozoa. It is obvious that 

 unicellular Protozoa can show nothing corresponding to 

 sexual reproduction in the first sense. Moreover, Pro- 

 tozoa can live on, dividing and multiplying, for prolonged 

 periods without the occurrence of anything like fertilisa- 

 tion. 



So it is often stated as a characteristic of Protozoa that 

 " they have no sexual reproduction." But if this means that 

 the Protozoa have no special reproductive cells, then it is a 

 truism. As unicellular animals, they can have neither ova 

 nor organs. If, however, the statement mean that the Pro- 

 tozoa are without anything corresponding to fertilisation, 

 then it is not true. For in the majority of Protozoa, there 

 occurs at intervals a process of " conjugation " in which two 

 individuals unite either permanently or temporarily. This 



