18-i PROTOZOA. 



place in the cell; their number is approximately constant in most 

 species; they exhibit extremely constant phenomena. The walls 

 contract and empty the fluid contents to the exterior, often 

 through a special duct. When one empties it completely disap- 

 pears and is formed again anew in a short time, and is filled with 

 fluid from the surrounding protoplasm. It thus resembles the 

 contractile vacuoles in the water vascular system (excretory organs) 

 of the worms to be described later. Apparently the contractile 

 vacuoles are for the elimination of injurious substances in solution 

 produced by the vital processes, among them possibly carbon 

 dioxide, like a respiratory organ. 



Apparently all the vital functions are under the control of the 

 nucleus. Experiments show that Protozoa, artificially deprived of 

 their nuclei, perform their functions incompletely and soon perish, 

 while fragments containing a nucleus remain alive. Young Pro- 

 tozoa usually have a single nucleus, and many have but one 

 throughout life ; but others early become multinucleate, Such 

 multinucleate forms are frequently regarded as cell complexes or 

 syncitia, but unnecessarily, for aside from the fact that in animal 

 and plant histology polynucleate masses of protoplasm are regarded 

 as cells, this term makes a distinction between the uni- and the 

 multi-nucleate forms, which does not correspond to the actual 

 relations, since the phenomena of both are completely alike. 



Reproduction is accomplished exclusively by fission or budding, 

 and under suitable conditions, such as abundance of nourishment, 

 occurs so rapidly that many Protozoa inside a few weeks can 

 number their descendants by millions. Many divide in the free 

 state while they are creeping or swimming about; others become 

 encysted before division. They become spherical and secrete a 

 protecting membrane around themselves (figs. 121, 122). En- 

 cysted individuals usually divide into more than two pieces, in 

 four, eight, or even many hundreds of reproductive bodies. It fre- 

 quently happens that multinucleate species divide into as many 

 parts as there are nuclei. 



In the Protozoa may occur a fusion of individuals conjuga- 

 tion which in many respects has much similarity to the process 

 of fertilization in Metazoa and in plants. In some (conjugation 

 of many Rhizopods) this does not correspond to true fertilization, 

 since only the protoplasm unites (plastogamy], while the fusion of 

 nuclei (caryogamy) necessary to fertilization does not occur. In 

 others a fusion of nuclei takes place. In the cases which have 

 been accurately studied there has been seen, before the fusion of 



