18 MORPHOLOGY OF PROTOZOA 



One chromosome of each pair was originally derived from the spermato- 

 zoon and the other from the ovum, and the two, or at least their descen- 

 dants, have remained distinct during all the subsequent divisions of the 

 nuclei. When the reducing division or meiosis occurs, the conjugation 

 of the individuals of each pair of chromosomes takes place, and it is 

 supposed that at this moment there is interchange of material between 

 them, and that transmission of hereditary characters is accomplished. 



It will be shown below that amongst the Protozoa the production of 

 gametes may be associated with similar changes in the nuclei, the gametes 

 possessing half or the haploid number of chromosomes. On the other 

 hand, cases are known in which no reduction in the number of chromo- 

 somes takes place during gamete formation. It results that the zygote 

 contains double or the diploid number of chromosomes. In these cases 

 the first division of the zygote nucleus is a reducing division, the two 

 daughter nuclei again having the haploid number. In the one case the 

 reduction affects the gametes and occurs before syngamy, while in the 

 other it affects the two daughter cells, resulting from division of the 

 zygote, and occurs after syngamy. 



GENERAL MORPHOLOGY OF THE PROTOZOA. 



Of the Protozoa there are a very large number of genera and species, 

 some of which are free-living forms, while others lead a protected existence 

 within the bodies of higher animals. The latter have undoubtedly been 

 derived from the former, and have become modified to such an extent in 

 adaptation to their hosts that, generally speaking, they are no longer able 

 to live apart from them. As practically every higher animal is liable to 

 harbour in its body one or more Protozoa, it is evident that the number 

 of parasitic species is very large indeed. It should be remembered, 

 however, that to understand properly the parasitic forms the study of the 

 free-living Protozoa should not be neglected. 



It is customary to regard parasites in general as degenerate organisms, 

 but though it is true they may have lost many of the organs possessed by 

 their free-living ancestors, they may have developed others in their place, 

 and reveal the same degree of adaptation to their environment as free- 

 living forms. Though a parasite may have lost certain structures which 

 it no longer requires, it digests its food, grows, and reproduces with all 

 the complexity exhibited by those which still possess them. It seems 

 incorrect to regard as in any sense degenerate an organism which is so 

 completely adapted to its environment as are the majority of parasites. 

 In fact it might be legitimately argued that if an organism retained 

 structures for which it had no further use, this would indicate a loss of 



